its just growing its second skirt, my guess its a fast growing paly and its body hasnt caught up with it yet.
alot of palys(zoos) go through mutation phases where they either grow new skirts,lose skirts,swell and change color.
here is a pic showing the 2 skirts on a similiar polyp
you can see 2 rows of 30 tentacles each.
your fine it will grow right evetually
