Strange creature in my Marine tank?

I believe it's called a bristle worm. I have no idea if I spelled that right. My husband has a salt water tank. I pretty sure they can be harmful to your fish as well. My husband had one in his tank and they are super hard to get out. I'm pretty sure they only come out when you turn out the lights. We ended up turning out the lights one night and using a flashlight to find him. He pulled him out with some of these long plastic tong things my husband has. I don't have a lot of info on the but, I'd look them up to see how harmful they can be. Good luck.