There's a lot of confusion about this. Fireworms (hermodice carunculata) are very, very rare in our hobby. They come from the Atlantic and around Ascension Island in the South Atlantic. Most of our stuff is from the Pacific. I have been at this for decades and I have never seen one except in pictures, even when I did have an Atlantic tank. So I'm a little skeptical of the fish store's id.
Fireworms have luxuriant huge pale tufts of bristles, and a conspicuously broad back compared to any other bristleworm.
They're not the only bad worm: oenone fulgida preys on clams.
And eunicids are the other large and troublesome sort of worm.
This is a link to a very amply illustrated RC article on Bad Worms.
http://www.reefkeeping.com/issues/2003-04/rs/index.php
Understand this, however: if something died and you see a worm exit the shell or skeleton, generally thank it, don't kill it---because worms are encouraged in our tank to be undertakers.
Bristleworms, hermits, and nassarius snails always come to the scent of something starting to rot, and they will clean it up for you before it causes a tank crash. That's why you should
never, ever, ever eliminate the bristleworm population from a tank. They're a little like surgeons, and can even help an ailing coral survive a water crisis, by eating off the dead tissue (proper bristleworms cannot possibly eat healthy tissue: they literally suck up the slime of decay, which is their food, and their mouths can only suck, not gnaw.) If I have a coral in trouble, I am glad to see bristleworms at work: they operate delicately, and sometimes the coral can then go on healing, now that the marine equivalent of gangrene has been sucked away to leave healthy flesh.
Nor do they sting, really: their bristles have little hooks, and the ridges on our fingers catch those and embed the little calcium carbonate bristles in our skin: hard on the poor worm, and hard on us. Vinegar will help dissolve those, and hot water on the site of the 'sting' will ease the discomfort. This is a good reason for us to wear gloves when moving rock. If a fish turns up with a face full of bristles, don't panic: they fall off after 3 days and the fish learns not to mess with bristleworms. I had a firefish take 3 tries at learning this---but he did learn.