Different mushrooms grow better during certain moon phases, some mushrooms also grow worst during certain moon phases. There is always a moon phase it is what the actual moon phase in the real world is. We have had 4 phases since mushroom farming has been introduced. We caught the tail end of Last Quarter, then we had Waning Crescent, New Moon, and we are in Waxing Crescent which will shortly become First Quarter. N/A moon phase just means the moon phase doesn't help or hurt the growth of that particular kind of mushroom. Ingame if the moon phase doesn't effect the result, it just says nothing about it in the result dialog.