IMO, the biggest bristleworm I've ever seen! Definitely a bristleworm. Glad you found someone to take it, that makes me happy
I have a lot of critters in my tank, I've found most of them to be innocuous. The only offending ones are a pair of Aiptasia (I've witnessed one attempting to eat a peanut worm) and a few hydroids (the digitate type went away on their own).