Is this what I think it is?

Despite popular belief it can be tamed. It does look like some abused anthelia. The method of reproduction is individual polyps drop off and let flow take them where they anchor themselfs using stolon, which is a mat made of sand and dirt that is kind of grown in that mixture.

Just keep the mat from forming, pick up and toss the pieces you see floating around the tank. But it will cover that entire rock in a matter of weeks if you don't keep it in check.

 

You can take out that rock and remove it I think. Can you get up under the mat and pull off? I'd try and scrape as much as you can off (with a razorblade) and out of the water and then put kalk paste over top to make sure it's dead.....that's my .02

 

How can it be tamed, besides magic? How do you keep the mat from forming?
How do you pick up and toss pieces floating around the tank at random times that get lodged in rocks where you don't see them until they explode in growth?

If you have some way, I'd love to hear it. It's a beautiful coral and looks great grown out. I'd love to be able to keep it - but it dominates and isn't cool enough to make a dedicated tank for it...