When I first started reef'n i bought a liverock that had 3 mushrooms on it. In a few months my rock was covered in algae and I didn't know what to do so I removed the rock and scrubbed it off. I sacrificed the 3 mushrooms cause I couldn't stand the rock covered in green hair. I scrubbed it clean, no sign of mushrooms or of algae and put the rock back. I guess I left little bits of mushroom cause they grew back! The rock now has 8 or so on it..lol So ya, they could come back.......or not.......