Any ideas?

[Joannez]

Hello everyone... I was hoping I could identify what is growing on my livestock. I was cleaning my tank and noticed this on the underside of the rock. Its spread on the entire underside of the rock. I'm thinking a type of sponge but was wondering if anyone can confirm or point me in the right direction thanks!! Your help is much appreciated!

 

[czwicky]

Sponge. Free filtration . May die after awhile because they have to eat ALOT to survive but definetly not a bad thing to have in your tank