The Moon is the most important 'planet' in relationships. The Moon represents deep security needs, and it is difficult to be with with someone who does not offer us what our Moon needs. Positive Moon compatibility will show two people who feel "at home" with one another, and instinctively care for one another. It's like being family. That's why it's the most important planet in synastry. If you want to spend the rest of your life sleeping in the same bed as someone else, you obviously need to feel like family to one another.
Moon Square Mercury
You do not tend to feel receptive to your romantic partners thoughts, statements and attempts to communicate. Whatever is being communicated is being hindered by your mood. This represents an emotional challenge to your partners ability to converse with you. When the Moon person is square the Mercury person, you will have a difficult time getting your romantic partner to listen receptively and are very likely to feel that they does not value your opinions or respect your intellectual capacity. The reality is that what you are saying or the way you are expressing it does not satisfy your partners present emotional needs.