That was my first thought as well, if it has a sort of small shell on it's back, it is a stomatella. Hard to see in this video, but that would be my first guess based on size and shape.
Maybe someone with a computer will have a better idea - I'm having a hard time seeing what it is on my phone screen. That video makes it look a lot less like a Stomatella in my opinion and potentially some variety of nudibranch possibly.