THIS CRITTER IS KILLLING ME. It's a sea slug that has converged on a fish shape.
Here's a vid of it swimming https://vimeo.com/159589650
It's "pectoral fins" are Rhinophores, basically antenna for smelling. It hunts jellyfish, and uses the remnant of its Foot to stick to them.
@Daylight I'm glad this one's a lot more cute and less scary