Please tell me what is growing on my rock

[nygator79]

Hello, I have had my tank running since July and came back from vacation yesterday to find this growing on two of my rocks. I am not sure what it is and was wondering if someone can ID it for me and let me know if it is something to worry about. Thanks

 

[Jlsardina]

Looks like a Sponge?

 

[zacheyp]

i think some type of sponge

 

[nygator79]

I would have never guessed that. Where would a sponge come from? It is starting to form on a second rock too. I am assuming then to just leave it alone as a sponge is harmless?

 

[Jlsardina]

Could have been a hitchhiker on your rock. I have the same thing