no disagree, they are some sort of comensal (parasite?) flatworm living mostly on sacrophyton leathers. been looking in my books and magazine's because i have seen a picture with name in one of thembut of course can't find the bloody thing. i have also seen one on a xenia when i didn't have any leathers (supsitute host ?). they blend in perfect with the host they live on and are very hard to spot. at night they will extent very long sweepers, which are pretty cool to look at with a torch, they expand them and then retract them with a short time. the sweepers themselve look pretty cool too. it doesn't seem to do any damage to the coral it lives on and if you really don't like it then you can easely remove it with some tweezers. i would take it of you if chuck it away. they seem to be filder feeders of some sort.
i only ever had 3 of them and that of a long time of reef keeping. they seem to vanish after a while, may be just enough food