What IS this??

[Sarahbell430]

Just noticed this yellowish whitish stuff under a live rock shelf there are four of "them" looks almost like cotton

 

[AlSimmons]

They're probably just harmless sponges.

 

[SALTYwaterDOG]

+1 probably sponges,

 

[ichthyogeek]

+2, definitely sponges.If you can run a finger on them, they should feel spongy, I got them a little bit before pineapple sponges outcompeted them

 

[bowhunterj]

I get them all the time esp. in overflow and sump hopefully harmless , I think they are good filter feeders.