Black algae? Or black sponge?

[Brett Stricker]

This black sheet of stuff has been growing on the back of this rock for a long time and we aren’t sure what it is... it is very slimy too.

 

[Oldreefer44]

I had something similar and it was a sponge that gradually started to takeover the tank. I would either isolate or get rid of it.

 

[Brett Stricker]

I had something similar and it was a sponge that gradually started to takeover the tank. I would either isolate or get rid of it.

Can’t get rid of the rock because it has a large coral on it.. does it help the tank in anyway?

 

[jeffchapok]

Sponges are filter feeders, so in that regard they help keep your water clean. As long as it's not an eyesore to you, I'd leave it be.

I have a number of similar black sponges in my tank, although none as large as yours. They all grow in the shaded undersides of rocks, so you have to try to see them.