Can someone tell me what this is?

About two years ago I bought a piece of coral that was attached to some live rock. On the bottom of the live rock was a white substance I just assumed was epoxy or something. Over the past year I have noticed the white substance is encrusting the rock it was on and other patches have shown up in the tank. Does anyone know what this is?

 

That looks like a sponge

 

I agree, sponge.

 

That's what I'm thinking but hoping for a more exact ID.

 

There are so many types that you probably can't get an exact ID without looking at it under a scope, I dunno.

 

There's not really any resource for exact ID's on sponges that I've found. You could start here and see if you run across it.

 

Yea I believe its a sponge as well. I have a ton of them. Completely harmless.