Is this a black sponge?

[BantyRooster97]

If it is, is it bad? It seems to be growing very well in my tank.

It came on a piece of rock I picked up at a LFS with the zoas on it.

 

[KJ]

Yes it is a sponge. Most sponges are harmless and some can get invasive and overgrow your corals.

 

[BantyRooster97]

thx. I have several others that live in the tank (blue, white & yellow) but none of them branch out like this one. & it grows fast