Both Moon and Mercury describe your state of mind. If they go well together, meaning they are in sextile, trine or conjunction, your feelings and logic work well together, if in square or opposition, not so much. Other aspects influence how it comes out as well, as always.
With Moon entering Pisces, the sign of the dreamer today, things can get foggy if your Mercury is in Sagittarius, Gemini, or Virgo. These are minor aspects, as anything with the moving Moon. Things get much more ‘foggy’ if Mercury or Moon are aspected by Neptune. They are also longer term.
Pisces brings vision and harmony, but also delusion and escape into fantasy. It’s ruled by Neptune, the ancients’ god of the sea. If you feel like your mind is in the midst of a whirlpool, check Moon, Mercury and Neptune for starters. And for real full picture, you need a full birth chart with all your planets and how they interact, drawn for you place, date, and time of birth.