no it won't harm fish, whilst its small I'd leave it be. its effectively part of the clean up crew. when it does get too big if you are lucky you can get it in the open with a pair of tweezers or if its in a rock take the rock out and then get the worm out. if i am right i think the bristles can irritate if you touch them.
I found one in my tank that i estimate to be 5 inches. the LFS said I should get it out immediately because I have an Orca Nano... but I really dont think its that big of an issue... it hasnt attacked the polyps I got last weekend. I think these things have a reputation that may be undeserved. They're great detrivores and sand sifters. Besides, i'm a bit afraid of being stung so i'm in no hurry to get at it with tweezers
There is a thread called Dr Watson and he identifies worms if you send them to him. I would leave it. Get a boxer shrimp they eat them anyway. We have left all of our worms and they haven't taken over the tanks, so something (as well as the shrimp I expect) is keeping my wormies controlled.