I have never heard of a bristle worm eating any fish that was healthy. More likely the fish was sick and dying and they did their job and cleaned up the carcass.
If you get to many, Arrow crabs love to eat them. Fun to watch them stab them and then tear them apart and eat.
Their bristles do not cause me much pain, but they sure make my fingers swell up! If you touch one you will get hundreds of very fine bristles in your fingers. The best why to remove the bristles, is to soak you fingers in vinegar and dissolve them.