What is this white stuff?

Whatever it is I am more concerned if it is going to cause any problems for the Coral it's growing all around?

 

It is a sponge and it will most likely prevent the zoas from spreading to where it is at.

 

Sponge?

edit: 2in10 beat me to it

 

It's a very cool looking sponge. I would be hard pressed to take the tooth brush treatment to it. But 2in10 is right the zoanthids will not fare well if you do not remove it.

 

Sponge? there sure is a lot of different kind of sponge huh. So the Zoas will be in no danger then I gather? So would you just leave it be?

 

Lots of sponges and yes the zoas will be fine.

 

I once had a sponge smother my green star polyps to death. Good news for you is that its not the same kind of sponge, as I can see from the picture. Regardless, I would get rid of it. Toothbrush treatment, or point a powerhead at it. They hate high flow.