It's got a giant blob foot full of what I can only assume are tapioca balls from a bubble drink, plus some pretty insane tentacles. Even marine biologist David Shiffman was perplexed, which is why he sent out a tweet earlier today asking for somebody to identify it.
Luckily, the internet had a ready answer. This is a Tethys fimbria, the only known species in its genus. They are creatures closely related to sea slugs that live in soft sediments of the Mediterranean region, and they can swim pretty well. Though I was hoping they were manta ray-sized, they don't grow much bigger than 30 cm across. Which is still kind of big, but not on the Cthulhonic scale.