Moon Crystal, a Japanese NES game, likely incorporates fantasy or science fiction themes with a prominent moon or crystal element in its story or gameplay. It could be a role-playing game (RPG), an action-adventure title, or even a puzzle game.  The game's setting could feature a lunar landscape, magical crystals, or perhaps a combination of both.  More information is needed to fully define the game's genre, plot, and mechanics.

Moon Crystal (Japan)

Monster Party (USA)