If you mean the purple one in the back, that is a cespitularia. That arm is the 'grabber arm', it grows them and they swing around until they connect to something, and they stick and grow from that new spot.
Its one of my favorite soft corals, they come in so many beautiful colors, but they spread incredibly fast. I don't keep them anymore for that reason. The best thing you can do it cut those grabber arms off with scissors when you see them.