Here's a thread of mine from a long time ago, before I knew how dangerous are these worms (to SPS). This is DEFINITELY a Bearded Fire Worm. (He went on to kill about 30 SPS frags, 1 each night, before I figured out what was happening!)
I'm sure there have been plenty of posts about bristle worms but in case anybody has not seen a FIREWORM, this guy is pretty fascinating! I've had tons of bristleworms over my 50+ years of reefing but this is my first Fireworm....probably because I've never used Caribbean live rock until...
www.reef2reef.com
Florida Keys are notorious for harboring these guys. I found that 2cc Ivermectin 1% in 30 gallons (rock Q-tank; not Main tank) will stun these worms in about an hour or two so they come out of the rocks enabling them to be caught and removed. (It is possible that the babies may not, though, because I found some in a rock years later when I split the rock open). You can then do a 50-100% water change and spare all the "good" worms. Highly recommended for Keys rock!