These guys are all over my tank. I have looked everywhere and asked many people, and no one seems to know what these are. They seem more active at night and when trying to touch the tentacle it retracts. I know it is definitely not a bristle worm or vermitid. And I do not think these things move. They might move but I could never tell. Any help would be appreciated because they may be stinging or irritating some of my corals. The tank has been up for almost two years. Thanks for the replies!