bristleworms get a bad rap. Chalk it up to being in the wrong place at the wrong time. They are scavengers. They will consume uneaten food, fish waste as well as dead fish/inverts or dying corals. While there are some worms like the fire worm or medusa worm that feed on healthy corals other common bristleworms will munch on dead or dying corals. Since they are often the first to the scene of a dead fish some attribute the worms to the fish's acutal demise.
Their population should be controled by the food source. Lack of food = lower bristleworm populations. Abundance of food = abundance of worms.