Looks like colonial hydroids. Either myrionema or athecate. These are stinging animals that can cause havoc with corals or clams in the tank if left unchecked. Control them by lowering the nutrients in your tank. Don't overfeed, keep up on water changes, etc. Otherwise, manual removal will be necessary to help rid the tank of them. Wear gloves while messing with them or around them and wash well afterwards. Use a small brush (tooth, nail) to brush them from the rock while using a piece of airline to syphon them out of the tank as you go. HTH