Dude I had the same thing happen. It looked just like that. It spawns new heads that detach from the original and don't form the normal skeleton structure you see with torches / hammers. It's grown a lot since but I'll snap a pic later
I think Jesse is on the right track. If it has a foot like an anemone then I would bet it's a majano. Best to get rid of it or put it somewhere the you can keep it from spreading in your tank.
Here's a majano with similar markings around the oral disk.
U may have got the frag plug from someone that had a hammer spawn or even water from someone who had a hammer spawn in there system. All it takes is one spore to land on a spot and start growing and develop into a coral. I have like 20 hammers popping up around my tank and I never saw one of my hammers spawn. So it can happen with out someone even knowing.