What is growing on my Favia? Good or Bad?

[eep013]

Anybody have any idea what this white stuff is?

It's growing on the side of the coral that doesn't get much flow. So far the coral still seems healthy. It eats when I offer it food and its tissue hasn't receded at all. Should I be concerned? The mystery substance is only on the rock, not on the coral itself (but it's slowly spreading).

Thanks for any advice.

 

[lorawalters13]

It is sponge. Should be fine.

 

[duec22]

Yeah, it's a sponge. The only thing I would warn you about is that they can really cause ditrius to build up. I've got a sponge that grows very simillar under my favities, and when I was giving the tank a really good cleaning I lifted it out and when the sponge tore off the rock, there was a heap of ditrius under it.