I'm guessing that it is a Eunicid family worm. Many people refer to these as bristle worms, although they are quite different than the traditional bristle worms that we all have.
Distinquishing characteristics of the Eunicid family are 5 distinct "antennae" surrounding the head/mouth. They also typically retreat into rockwork with lightning speed when disturbed. Yes...centipede like legs.
While some Eunicids can grow to enormous length and are the stuff of Aquarium lore (Oregon Reef Eunicid over 7 feet long...), others are harmless detritus feeders. Keep you eye on it and see what it eats.
Here's one of my favorite photos of "Max" who lived in my 30 gallon hex for 5+ years and was probably over 2 feet long. He was strictly vegatarian.
Oh yes..I forgot...really nasty jaws that they use to grab and retreat.
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400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank:
http://www.canreef.com/vbulletin/showthread.php?t=28436