What is Living in My Zoas?

[LPStandard]

Spotted this white stuff under some polyps while feeding today. The frag is always open, doesnt look unhappy at all.
Sponge? Something more dastardly or nefarious?

 

[SAPPER]

Looks like a patch of Clathrina sponge growing. It might not be a problem but it can cover corals so just keep an eye on it.