I found this in the filter. I took rock from and existing sump to start a tank. I’ve never seen it in the other tank. Almost looks like the inside is filled with sand and has these stringy tentacles on both ends. . I don’t have sand in the tank.
Looks like a spaghetti worm. They will wrap themselves with sand. Some folks say they might bother corals, but I've never had a problem with them. They're just another member of my CUC.