some run of the mill palys.... I would either try to pull them off, and keep them on their own small rock isolated from other rocks, or simply nuke them now. Those things can go wild & take over your tank. They will sting any coral next to them too. If you decide to keep them, once they run out of room to expand on the rock, they will let go of a couple polyps at a time, which will float around, landing on nearby corals or rocks until they lodge into a crevice & start growing again.