another what is this

[7808]

noticed this growing on most my rocks, un the underside or un-lit sides.

 

[piercho]

Pretty sure its a sponge. In fact I think I see 2 types of sponge; one composed of thin hollow tubes, the other has a pear-shaped body.

 

[7808]

is it good or bad?

 

[mrferrit]

filter feeders there good for you tank i have a bunch under alot of rocks

 

[lancer99]

Keep an eye on it. Most sponges are, as mrferrit mentioned, harmless filter feeders, but there are some that are invasive, and that looks like one of the invasive species to me.

-R