It looks like a leather coral, probably a toadstool leather.
They can get very big. I had one that was 14" tall and 8" in diameter at the top.
When it gets bigger than you want, you frag it. They are super easy to cut with any sharp blade or even good scissors. When cut they will slim like crazy so have a container of extra saltwater to put it in for awhile after you cut it. The cuts are slow to heal and leathers are very slow to attach to frag plugs or rock. Many people use a variety of methods for holding them in place while they attach. I use a big sewing needle and sailcloth tread and put 1 or 3 loops through the coral and around the rock or frag plug.
You could try trimming a piece off it now while it's still small. The mother colony could stay in the tank and heal there. It only takes a week or two to heal over, after a couple of months you'll be lucky if you can see where it was cut.
Have fun.