The Moon is a feminine planet, which rules our emotions and domestic life. Mercury represents our communication style, and is genderless.
When your Moon makes a harmonious aspect (conjunction, trine, sextile) to your partner’s Mercury, you are attuned to your partners’ thoughts and feelings. As such, this aspect is great for communication! The Mercury person has the ability to verbalize the Moon person’s feelings.
The Mercury person is also a great counselor for the Moon person, due to the fact that the Moon person feels comfortable expressing her feelings to the Mercury person.
The hard aspects (opposition, square) between Moon and Mercury are a bit more complicated. In this case, misunderstandings and confusion plague the couple. The Moon person is highly sensitive to the Mercury person, and tends to take whatever the Mercury person says very personally.
Mercury is a logical and emotionless planet, and the Moon is tuned in on an emotional level, so this aspect indicates the two are not speaking the same “language.” Indeed, the Moon person may find Mercury insensitive and overly-objective, while the Mercury person may find the Moon person over-sensitive or moody. The Mercury person may feel they are walking on eggshells when they’re around the Moon person. This lack of emotional understanding can lead to arguments and debates.