Please tell me what is growing on my rock

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nygator79

Aquarium Advice Newbie
Joined
Jul 9, 2011
Messages
4
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
 

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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?
 
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