I am a novice when it comes to marine biology, but I do have an interest and I think cephalopods are cool. Looking at this post I am fairly convinced that the mystery creature is tremoctopus. Here are some visual comparisons of the photos already presented on this post that helped me with this conclusion:
Swimming style of Sea Hares:
They seem to have a very typical body shape and swimming behavior that does not look similar to the mystery photos. The mystery animal seems to lack the large, skirt/wing like appendages. Also, the sea hares seem to keep elongated for the most part with thinner structures both forward and behind the animal, features also apparently not present in the mystery animal.
Also notice the color patterns, they are very mottled and there is rarely a smooth coloration even on a portion of the sea hares, unlike the smoothly colored animal in question.
The portion of the sea hare that is mantle like in appearance, as far as I know, lacks the holes visible in the mystery animal.
More Texture and Morphological Differences from Mystery Animal and Sea Hares:
The stalk like appendages do not appear in the mystery photos as far as one can see. The animal could be inverted.
Similarities Between Tremoctopus and Mystery Animal:
Notice that the mystery animal, when alligned with a top view of tremocto, has a similar proportionality in features. This could be flawed to some degree in the comparison due to different perspectives in the photos and the size of the animal in each is unkown. However, the two holes are present in both, coloration is smooth and similar, and other parts of the mantle seem to be a match for the mystery animal. Swimming behavior of both also seems alike.
The big difference is the lack of tentacles in the mystery animal. It seems to me that they could be tucked-in in the mystery photo. Using Occam's Razor, it seems easier to me that this would be something similar to tremocto with tucked or missing tentacles instead of a sea hare that has lost its wings, is swimming oddly, has tucked or lost other anatomical features, and has a rare coloration.
As for nudibranchs, the branching structures seem to be absent in the mystery animal, and again, coloration and wing structures are also not visible.
This is the weakness of relying soley on photos. You cannot always get a descriptive angle and the animal may not be in a typical pose, meaning one can draw many conlcusions and imagine just about anything
Goodluck, I really look forward to the verdict!