Is this a sponge, or worse?

[Sbpe]

I just noticed these things recently popped up on the underside of rock in caverns and other low light areas. There must be a dozen or more now. They are white/grey in color and kind of translucent. Is it a type of sponge or something more nefarious? My reef tank is about 18 months old biocube 32. Any thoughts appreciated! In the photo there is one on the underside of top rock and one directly below it.

 

[Billdogg]

encrusting sponge. Filter feeder. IMHO, a beneficial part of your CUC. Once fully established you will begin to see other colors as well. I've got yellow, green, blue, white, red/pink and black sponges growing here and there in my LR and sump.

 

[ahiggins]

Yep sponge

 

[Sbpe]

Thx, I thought they might be, but it concerned me that they looked a little slimy. And was surprised at how many there were when I looked around good. Will they keep to the low light areas ?

 

[vetteguy53081]

This is a Poriferan sponge and can become invasive and overtake your coral. Hard toothbrush or cutting away will stop it.