I just saw one of these in my tank today. I tried to take a pic but as soon as it heard my camera's lense motor, it shot back into its cave. I read that it can retreat at 20feet/second.
http://reefkeeping.com/issues/2003-04/rs/index.php
"Contraction back into a burrow has been clocked in excess of 20 feet per second, and if only a couple of feet of the worm are visible while it is foraging, that worm can disappear, quite literally, in the blink of an eye."
There's a picture of one in the above article where the author named it Max. I wonder if it's Untamed's pictures?