Sounds like Bristle worms. Do they look kinda fuzzy? They will not hurt nothing, and are considered beneficial to your sandbed. I have seen stores selling them in their live sand start up kits. Can you post any pictures? Also for me if I touch them my hands/fingers swell up some for a few days. I have to be careful when moving liverock in my system because of this.
it looks like a bloodworm. Its not fuzzy, I have a few bristle worms dont look nothing like it. Ill try to get pics
