I like it! But like the others said, I think the other planets would be a little left out if only Relcore got the moon. But here's an idea:
Maybe the moon could move to different planet once a week, and there's different creatu on the moon depending on which planet it's at, at the time. Or, there's only one creatu on the moon and it changes depending on which planet it's over.
(I.e. If the moon is over reiflem the creatu on the moon is orange, red colors and kinda in the form of fire. Or if the moon's over Atquati, the creatu on the moon is in the form of water with a blue color)
But, my idea kind of reminds me of eevee in pokemon, changing it's form when it eats the certain stone or whatever...