What is this, and what should I do about it?

[The sea slug]

I got this acan a little while ago, and I noticed that it wasn't growing. It wasn't losing heads, but the ones it had weren't replicating. Then a little while later, I noticed this stuff on it--it's milky-white and porous, like maybe it's a sponge, except it's grown all over my acan. What is it? And what do I do?
Other tank inhabitants include a banggai cardinal, a clownfish, a damselfish, a colony of zoos, and a duncan coral. Water parameters normal.
Thanks guys

 

[D-Nak]

Yes, it's a sponge. If it appears to be irritating the acan, you can use tweezers to carefully peel it off.

 

[The sea slug]

thank you!!

 

[azianreefer]

it's a sponge I agree...you can get an angel fish it'll eat the sponge or manually remove it