My guess is something dying. I remember when I fragged some mushrooms and once in a while one got away. My plate corals are right underneath the mushroom garden. The flat discomosa mushrooms seamed to notice the plate and vice versa, but didnt really do an real damage warfare between the two. When I went to frag some hairy mushrooms one landed right next to it. I didnt think it would do much damage at the time so I was in no real big hurry to move it away. I was wrong. The hairy mushroom formed a kinda film on it and died shortly after.