I think it might be a sea hare, a type of sea slug:
Sea hares have large flaps covering their backs and gills. They also have large 'ears' for which they are called 'hares'. They also have large mouth flaps with which they can explore a wide swath for edible weeds. Being so vulnerable without a large protective shell, they are nonetheless not predated upon, presumably because they taste foul. All sea hares are voracious grazers and prolific spawners. Like octopus, sea hares can squirt ink when frightened. All sea hares carry a remnant of a shell under their folds. By opening the folds gently while sliding one's finger over its skin, one can feel the thin shell. If the sea hare co-operates, it can even be seen.I'm not sure if it's a sea hare, the colors and body shapes seem to vary greatly, but you can really sea the "ears" in the photos, and what appears to be flaps on its back.
Here's some video of one munching on some algae: