This is a real creature. At the Reddit thread, after some preliminary speculation that it was a plastic sheet or other debris, or perhaps the placenta of a whale, it was identified as a Deepstaria enigmatica. It's a cnidarian. You learn something every day. I am repeatedly gobsmacked to discover the amazing creatures we share this planet with.
Addendum: Here are two relevant comments from the thread at BoingBoing, one from Jonathan -
I didn’t initially think it was a jellyfish either, but around 4:46 in the video you get to see the mouth on its long, pendulous manubrium hanging from the center. From the base of the mouth, you can see the extensions of the gut radiating symmetrically outwards, where they meet the gonads (the white masses). This is a great diagnostic to identify it as a jellyfish, whatever the shape of the bell.- and from Jellywatch:
Deepstaria is not that rare, but is large (more than a meter diameter), so rarely seen intact. In the video, the swirling from the sub makes the medusa appear to undulate and it even turns inside-out. Normally they just hang out like a balloon.And finally, a hat tip to Robb for finding a report on this video at RT which includes screencaps of the creature (useful in case the video is taken down again).
Second addendum: The video embedded at the top is interesting, in part because of its ambiguity. But that ambiguity arises because the creature is being pummeled by outwash from the ROV engines, rather than behaving in a normal manner. In response to this video, Steve Haddock, a research scientist at the Monterey Bay Aquarium Research Institute, has posted a proper discussion of Deepstaria and its near relatives. Beautifully filmed, and intelligently narrated --
With a hat tip to Mark Frauenfelder at BoingBoing for the via.