I agree with above, the are safe in the tank and mostly will not hurt anything and most of the time, only yourself or someone try to work on your tank might get hurt if you accidently get in touch with the worm with your bare hand.. you can feel the sting but won't kill ya...
Some bristle worm eating fish will help.. I think six lines hunt those for dinner
Yes it's a Bristtle worm, but they don't sting per say, but they have small stiff spines that break off if you touch them, and they get embedded in your skin. They can cause irritation for up to a week until they flake off with your old skin.
Bristtle worms are good for your reef, because they are good cleaners. They come out at night and eat your uneaten food and fish poop, or whatever else that will mess up your tank's chemistry. I believe they also will help keep your live rock cleaned out, so they do become stagnate, or clogged with stuff. If you come out with a dim flashlight at night you might see a lot of them, espesially if you bait them.