Milky white substance growing on my rock

[NOLAdoogie]

Hello. Just notice this strange substance on the backside of my rock work. I have two RBTAs perched on this rock, one of which split a few months ago. The substance almost looks like a dead RBTA I didn't know I had (again it's been behind a rock and I don't know how long it's been there). Whatever it is, it's translucent as seen in the picture.

120 Gal with 2 clowns, 2 tangs and 3 RBTAs. Besides battling the occasional Green Hair Algae problems, the tank has been doing fine and I've had no casualties. PH and alkalinity is low but I'm working on that. Salinity at 35 and temp around 79. Near zero phosphates and nitrates.

What could this be? What's causing it? What's the best way to prevent/get rid of it?

Thanks in advance.

 

[SHNICI]

it look like a sponge.
Google: reef rock sponges, and in photos you will find some matches to your one probably

 

[Pete80]

That’s a sponge
its beneficial

 

[vetteguy53081]

poriferan sponge which can potentially grow out of control. Should it do so, You can trim it or inject air or vinegar into it which will control it

 

[JGT]

Hello. Just notice this strange substance on the backside of my rock work. I have two RBTAs perched on this rock, one of which split a few months ago. The substance almost looks like a dead RBTA I didn't know I had (again it's been behind a rock and I don't know how long it's been there). Whatever it is, it's translucent as seen in the picture.

120 Gal with 2 clowns, 2 tangs and 3 RBTAs. Besides battling the occasional Green Hair Algae problems, the tank has been doing fine and I've had no casualties. PH and alkalinity is low but I'm working on that. Salinity at 35 and temp around 79. Near zero phosphates and nitrates.

What could this be? What's causing it? What's the best way to prevent/get rid of it?

Thanks in advance.

Spongebob. Good guy.