Eunice have no legs just bristles the same as Bristle worms. Those are bone jaws at it's mouth. It uses them to lunge up out of the sand and grab unsuspecting fish, which it then rams against the sand until the fish is smothered and choked by the sand.
Eunice worms are known as Bobbit worms. They get their name from their latin name: Eunice aphroditois
"Fireworms" I believe are what people refer to as just bristle worms. They're the red/pinkish variety which we have in our tanks commonly. They only eat some non-photosynthetic corals such as Gorgonians and some starfish as they never venture out into the light where light gathering corals live as they'd be eaten by fish. Those are Hermodice carunculata.
But there's thousands of polychaetes and so there's a big blur in identifying them, where one you have might be a slight variant of one of the main types. Tube worms are also bristle worms, and there's many types of those as we know...
If it's a bristle worm, I'd leave it alone until it got massive, where it's appitite is going to go beyond scraps of food it can find and it might pounce on sleeping fish, especially those vulnerable like six-lined wrasse which cocoon themselves around the top in a bubble of mucus but are still, and if anything more vulnerable from beneath.
If it's a Eunice worm / Bobbit worm, it's got the power to do this, so I'd get it out as soon as you can.Eunice worm in action (graphic)