Identification please. Web on live rock.

[Cornishlass]

I have dry rock curing in a tub. I've added a small piece of live rock to seed it all. But I have noticed this weird web like structure on the rock. What is it please and is it bad?

 

[j_mazzy]

looks like a cool sponge

 

[Cornishlass]

Is it harmless?

 

[mcgyvr]

yep.. sponges are good "filter feeders"