The chaetae visible by the tail eliminates sea cucumbers and a few cucumber-like worms. From what I can see, it could be a few different things, but it reminds me most of the "Blue-Tailed Syllid" worm (if you click by my name in the quote below, it'll take you to the thread the quote was pulled from that has a pic you can compare to - there are a few other pics floating around on the web you can find too, but there aren't a whole lot):
So, it may be a syllid worm, or it may be an "Accordion Worm" (which I also can't find a scientific name for):
http://www.underwaterkwaj.com/uw-misc/polychaete/Worm-accordion.htm