What is this??

[Darryn Lyon]

What is this mucus-y membrane looking thing covering this hole in my live rock? A brittle starfish lives in the hole... do they do that?

 

[Animagus]

Sponge most likely.

 

[Tahoe61]

Looks like sponge with a micro brittle stars tentacle sticking out.

 

[tomtom2245]

+1 on the sponge

 

[Darryn Lyon]

I would assume some sponges are beneficial and some you don't want in the tank?

 

[Tahoe61]

The vast majority of sponges are fine, there are few invasive ones.