What the H#ll is this?

I have a strange creature beginning to show itself in My reef tank. It looks like a purple catapillar or centepede is the best way I can describe it. If You sit very still during feedings it will come out of the LR but never fully shows itself. It appears to be several inches long and lives it the LR somewhere, I've tried moving the rock to see it but to no avail. The thing moves at lightning speed if it notices You, I mean faster than the eye can follow, one second it's there, the next it's gone.
Any ideas from anyone?
Latest update: This thing is over 12 inches long and has rather ominous looking jaws and will even try to grab small fish It is becoming a little bolder and coming out more. It likes shrimp pellets and can be lured out at night with them. It has created a tube under the sand which it lives in.

 

Try looking after the lights go out with a red filter on a flash light. It sounds like a bristle worm to me

J

 

I had the same thing in my tank. I was investigating if there was a mantis shrimp in the tank so I was especially watchful. And just as you said, During feeding I'm watching a piece of flake drift by and , WHAM, this little millipede looking thing was in and out with it in the blink of an eye and back under a rock. so I removed the rock while my wife watched to see that it stayed in the little hole, and then immersed the rock in DI water for about a minute and he came out.
I took it to the LFS who said it was a bristle worm and harmless to my tank, and it could have stayed, but with little purpose as a scavenger.
Another thing i noticed was these fast little bugs that run in the cavities of the rock at night (i used a LED penlight at night, and their eyes glow) which are copeopods (sp) and also harmless to tank, and are some times food for other fish, such as my mandarin Dragonet. 8)

 

You got your BW ID!