It's a bristle worm. They're a good member of your (cuc)clean up crew. They do get large over time, but don't pose a threat to anything really. Just don't touch them.
I had so many of those in my 12 gallon jbj cube... until I introduced a pompom and a pistol shrimp. I don't know which of the two destroyed them but the mass population is seriously gone. lol