What is it, how to control

[FlytheWMark]

This popped up on this rock a long time ago and grew very slowly so I didn't pay it much attention however its began growing like a weed about 2 weeks ago and has tippled in size.  A) What is it. B) How do I control it?  

[Joevember]

Looks to be an anthelia. It's hard to get it off rocks once it's attached. If it's possible, get the rock it's on out. You can keep it out or peel it all off. 

I've had anthelia in the past, and the polyps can eject from the colony and pop up on random rocks in the tank, so look around for some straglers too.

[Murphs_Reef]

Interesting stuff that.. defo need to pull the rock and scrub it.. 

[mcarroll]

Sell that rock or trade it for a "clean" live rock.

 

Lots of folks love that stuff.  If you can give up the rock to it, I agree that its sway does give a nice look in a softy tank.

[Tired]

I'd suggest selling or trading it, yes. It's a coral, which means it's not harmful in and of itself, it just has the potential to spread fast. Looks like the rock it's on can probably come out pretty easily? So you can probably get something for it, if you find a buyer. It's a nice texture, I'll say, though would be far too invasive for me.