Neptune naturally represents the lack of boundaries, while Moon represents our emotions, emotional needs, and subconscious wiring and responses. So what happens when you see Neptune square Moon or Neptune opposite Moon in the natal chart?
First, let's throw a series of keywords before we go in-depth in the explanation:
Neptune square/opposite Moon keywords: Codependency, emotional confusion, emotional incest, high dependency, lack of emotional clarity, unknown or suppressed emotional needs, emotional projection, high sensitivity, high empathy, high intuition, tendency to escape emotions (avoidance, numbing, escaping, projecting, deflecting), tendency for depression, high creativity, selflessness, unconditional love, longing for love.
Neptune square/opposite Moon often represents someone who grew up in a family where there was a lack of clear emotional boundaries between the primary caregivers and the child. Especially if the aspect is applying (Moon is a lesser degree than Neptune), the first years of life are tainted with a lack of appropriate boundaries. The child has a tendency to be confused with what belongs to them and what belongs to others emotionally. The child will have difficulties knowing what it is they feel, what it is they need, giving adults who often experience the same challenges.
There is a tendency to feel others, giving great empathy of course, as well as an excellent capacity to "read" other people and know what they feel and think, but unfortunately, this skill usually is difficult to use because the person is going to naturally find it challenging to spot adequately what is other people's emotions and what is theirs - ending up often projecting their emotions on others, adopting other people's moods, and finding themselves very confused with what they are actually experiencing and what they are responsible for. Typically we'd see for example, someone who feels angry and, as they find themselves unable to experience their own anger, disconnected from their emotions, will find themselves seeing angry people around them.
Of course, this breeds the perfect soil for codependency, a relationship pattern where one person becomes absorbed by the other, lose a sense of self to the benefit of the relationship, and becomes overly responsible and concerned with another person, who usually has addiction patterns. This is Neptune's territory and codependency can be seen through other aspects in a natal chart, but Neptune - Moon is a very common reoccurrence in codependent relationship patterns.
Codependency also involves a tendency to forgive, excuse, and overlook the partner's behaviours and responsibility, which is something Neptune - Moon, in its goodness and avoidance, is capable of doing.
With this aspect, because the person has difficulties identifying and owning their emotions, there is a tendency to project, deflect, numb, repress, escape, distract - from one's emotions. This gives a tendency to avoid painful emotions, but it also gives the person difficulties experiencing more appealing emotions, such as love. Sometimes, we find people who are constantly running away from their heart through various coping mechanisms, and often we find people who live in somewhat dissociated states, having difficulties grasping what love is, while looking for it desperately all their lives.
Neptune - Moon has high longings for love, and a tendency to be in a constant state of looking for more nurturance, more love. Neptune's hard aspects to Moon often point out to a childhood where the primary caregiver was somewhat unavailable, perhaps too absorbed by their emotional hardship, perhaps away physically, or away emotionally.
With this aspect, it's difficult to say how much is nature and how much is nurture. You will find that there is a certain degree of this aspect that represents the unavailability of the caregivers growing up, and a certain degree that describes a person's innate tendency to be highly sensitive and longing.
Whether it's nature or nurture, chicken or the egg, with Neptune - Moon we find someone who tends to feel alone and disconnected, and therefore someone who has a strong drive to connect and long for love.
Moon represents our emotional needs, our dependency needs, and our attachments. Someone with Moon - Neptune is going to experience difficulties with these themes, either losing themselves in utterly dependent bonds or finding it difficult to find a healthy and balanced approach of dependency. For example, if the Moon is simultaneously square Uranus, we'll see strong patterns of disconnection in attachments, while if we find Saturn square Moon, we'd find strong patterns of avoidance and shutting down, in attachments.
What I just described is at the root of pretty much any Neptune - Moon challenging aspect; however, it is not a curse, and there are plenty of ways to use this energy positively, as well as plenty of ways to heal this energy.
For one, Neptune - Moon people obviously have a serious advantage - it's that they have a sensitive and open heart. They have a strong capacity to penetrate others and know what they feel and what they need - however, this skill will be heightened and refined once they start the healing process of their relationship with their own emotions and emotional needs. The more one is aware and in touch with their emotional self, the more they are capable of being in touch with others' emotional self.
Another skill of this aspect is that one is easily in touch with the concept of unconditional love - accepting someone for all of they are, without requiring them to fulfill expectations or needs - this is something Neptune - Moon people can be capable, but once again, there usually is a need to work on the self first, as often what happens with this aspect is that, one suppresses or avoids their own emotional needs, and will tend to expect others to know and fulfill their disowned needs. Once the reclaiming work is done, there is a true capacity for unconditional love.
Neptune in hard aspect to Moon is going to benefit from reconnecting with the emotional self, reconnecting with the emotions, reconnecting with the body. Often, somatic work here is helpful as there is a tendency to be split, dissociated with the true feelings.
As Moon represents anything comforting, including food, we can find distorted food habits with this particular configuration. Moon also is the old part of the brain - the amygdala / the limbic brain, and so with Neptune-Moon, we can find a tendency for depression, and sometimes, a link between depressed moods and food habits. Occasionally, we also find digestive issues that don't seem to find a root or a solution.
When we see this aspect in the chart, trying to change the food habits or trying to find health solutions without addressing the emotional root is often a lost cause. The whole pattern of this aspect is to learn how to reconnect with the emotions and the emotional self.