I have this super weird worm ive finally managed to catch. Its white and seems to swim really fast by wiggling quickly. Its superweird and havent seen anything like it
Getting a good photo is hard since its never still.
Looks like the epitoke phase of a polychaete worm. (Bristle worms are one example of a polychaete, but it's a _huge_ group!) By itself, it's pretty harmless, and may be a good snack for a nocturnal fish.
If it was the epitoke of a bobbit worm though . . . (rather unlikely.)
Thanks Maritimer, that's what i'd gathered from trying to look it up. I've got it out of the tank in a specimen container. I'd rather remove it to be safe, as all my pest eating fish are early sleepers.