Ah, a sacoglossan! First one I've seen in CA, I think. I'm fairly sure this is Hedgpeth's Sapsucker (Elysia hedgpethi). It's not quite the dark green color shown in Behrens & Hermosillo (2005), but the book does say yellowish brown is a possible color variant, and it certainly has the tiny blue spots. I saw this stretching on a piece of brown algae, which I think is the only way I was able to see it. About 8 mm.