The opposition of composite Moon and Neptune can present many difficulties in a relationship, all stemming from one problem. First, of all Neptune-Moon combinations, this one is most affected by a desire to escape your own problems through a "savior-victim" relationship. Since neither of you will face your real problems directly, nothing is solved. You should try to relate as equals, giving and receiving alike, with neither of you martyring yourself to the other.
A second result of this aspect may be that the two of you have radically different views about the relationship. While one of you idealizes, the other may take advantage and act quite treacherously behind the first one's back.
The main problem is one of illusions. Detachment and objectivity are needed at every turn. Perhaps the best answer is to rely more than usual on a third party to help you come to a true understanding.