Maybe a sponge?

[Lilgrieves]

Trying to figure out what this is on my disco rock

 

[Gumbies R Us]

@ISpeakForTheSeas

 

[DSC reef]

Definitely sponge. Most are harmless filter feeders while some can suffocate corals. The ones that become troublesome usually can be removed with a soft toothbrush. That mushroom doesn't appear to happy but you know your tank better than I do. Those are beautiful mushroom

 

[Lilgrieves]

Definitely sponge. Most are harmless filter feeders while some can suffocate corals. The ones that become troublesome usually can be removed with a soft toothbrush. That mushroom doesn't appear to happy but you know your tank better than I do. Those are beautiful mushroom

lights were going down when i took the pic so the mushrooms were shrinking