it's waving hands(anthelia), congrulation for the hitchhiker.
At the first glance, I thought that's my rock coz the zoa are same color and anthelia are walking up the reef every here and there in my tanks.
Actually, they don't walk, just that they will slowlly stick to a more suitable location in adjusting to the flow and light. If you change the direction of the flow they will slowly stick to another location. A lot of the times, they will "walk" up the glass after couple months.