This is the swima bombiviridis, a bioluminescent “Green Bomber” that was found in deep Pacific waters in 2009 by Karen Osborn of Scripps Oceanography. From ScienceMag.org:
Thousands of meters below the sea, a tiny worm wriggles through the darkness, its dozens of paddle-shaped bristles moving in beautiful coordination. Suddenly, a hungry predator appears. The worm releases a glowing green sac, and the fish homes in on this bright new trophy. By the time the fish realizes the sac is no meal, the worm is long gone.