ID Help?

[Miza]

This greenish/clear stuff is growing on the bottom of one of my life rocks. Can anyone tell me what it is and if it's good, bad, or neutral? Thanks

 

[ritter6788]

Sponge. Not harmful.

 

[NanaReefer]

I have a lot behind my rock on the back wall. If you look closely you can see the little siphons on them. Pretty cool if you ask me. Harmless filter feeders


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