WOW<
You say its FAST?? Thats what confuses me?? Hmm
Maybe you could try to trap it. You can buy a bristle worm trap at your LFS for like 12 bucks. Mine trapped all sorts of stuff so you can use it for other stuff too.. BUT being that this is so big you will have to take the end caps off and use a nail clipper and make the entrance wider... Maybe you can catch it! Put the trap next to where you see it hiding and shut the lights off and in the am, maybe it will have caught it to get a better ID... If its good, let it go, if its BAD flush it..
I have a HUGE bristle worm and I cannot for the life of me catch it.l Also, I see another small bristle worm that outsmarted the trap.. So good luck!!! Please post and let us know.. I do re visit posts..
Thanking you kindly, Kim