What is this??

[Ashtina2688]

Found it on the underside on one of my rocks. White, solid, looks puffy. Thought it was a piece of shrimp but it's not.

 

[bang guy]

It's a sponge.

 

[Ashtina2688]

Thanks. Do they just show up? I've never seen it before. And we just recently moved so had to re set up the tank and it wasn't there a month ago.