Unidentified species

Can anyone identify this organism ? It was approximately 6 cm in length, photographed at 6 to 8 meters in depth at night off the coast of Tanzania. On the one hand it looks like a flatworm, but it is too thick to be a flatworm. It also partially resembles a Sap Sucking Slug but I cannot find anything that resembles it in any book or website. Sent it to a few people and so far no one seems to have seen anything like it.