White stringy sponge

[reefer10]

This white stringy stuff was the on the underside of the rock. Is it a sponge?

 

[20gallonreefer000]

Yes it is a sponge. I've seen this before and read through some threads about it. It seems to be harmless for the most part except for when it grows on or too close to corals. It also seems to be something that appears in newer tanks and dies off as the tank matures. Couldn't find a name but it should be fine in your tank.

 

[saltwaterfish.com]

Looks like a spaghetti sponge, they are usually harmless and will just filter feed in the water column.