I'm not sure of the name of those worms but they are filter feeders and no harm in your tank. BUT do I see aiptasia in there? Those you don't want. We had just a few in our sump and a couple monts later (now) we have them creeping into the Display tank. We can kill them in the display but haven't figured out how to get rid of em in the sump.
Can you get closer pictures of the stringy critters in the current pic?