Yeah, the Z-move button and Mega-evolution button occupy the same slot, and the Z-move takes priority (i.e. if you can both Mega and Z-move, only the Z-move button shows)
However, as far as I can tell, if you Trick a Z-crystal onto an already mega mega-Ray, he can then use a Z-move. Assuming Z-crystals can be tricked, bulbapedia doesn't say they can't, but it seems like something that could be excluded.