There are more misconceptions about polychaete worms than there are truths in this hobby, sometimes.
As stated before, bristleworm and fireworm are both interchangeable umbrella terms covering thousands of species. These particular words are fairly meaningless. Now, when most people say fireworm, they are referring to a specific species of bristleworm/fireworm with a common name of Caribbean Bearded Fireworm. This species preys largely on non encrusting gorgonians. It has a rather explicit means of doing so. It finds a gorgonian such as a sea fan or sea whip, crawls onto it and deepthroats a branch, gumming the tissue into goo that it can digest.
It does not pose a significant threat to other types of coral, or to fish. By the most scientifically accepted definition, what we call feather dusters are also just as appropriately called fireworms.
Very very few polychaete species we see in this trade pose any harm to our stock. Those that do are very easily recognized. As a general rule that applies the vast majority of the time, fireworms/bristleworms are the most beneficial organisms we can have in our tanks. I wrote an article that is on this site in the articles section, about this very topic. It includes pictures, scientific names, and citations for more clarification.
Long story short, that worm was a good guy and you should be pleased you have them in there.