Identification white stuff on rubble rock

[nimbus]

Can folks help ID what this stuff is? This piece of rubble was free of anything for months, I had noticed a ball anemone on another piece but in the last few weeks this stringy white stuff has formed over the edge

 

[Geraud]

Looks like sponges to me. Never had any issues with these. They always appeared in the non-exposed parts of the tanks.

 

[nimbus]

thanks!

 

[Josh]

It's a harmless sponge.

 

[nimbus]

Tank was started sterile, 15lbs caribsea life rock and eventually added 4-5lbs LR rubble to seed the system. Very cool to see these kinda critters pop up. Thanks