Please ID this

[jerrym21]

I have this growing on my rock for a few months. There is only one that I can see and it does not seem to be getting any bigger. I would just like to know if I should get rid of it or leave it alone

 

[zenya]

It's a sponge. They commonly called satelite sponges in the hobby and they are harmless if you are concerned). If I remember correctly they call that because they have ability to move from place to palce.
I'll look up a link for you to an article describing sponges, if you need it.
Just holler.