They are flatworms. Unless you actually see them on your corals I don't think they are the "bad" kind. I have had some living in my chaeto tank for a while and yours look just like mine. It's a Biocube separate from any other part of my saltwater system and there is nothing in there but a little sand, chaeto and pods, so they aren't coral eating flatworms. They most likely eat film algae since I rarely feed the tank anything more more than phyto or the odd algae wafer. I don't think you need to worry about them much less rid the tank of them.