Parasitic on dead Mola mola. About 1/2" long, vaguely looked like a tiny horseshoe crab with two tiny claws and a bunch of legs on the underside. Photos show dorsal and ventral views, and what it originally looked like on the mola's skin.
Getting any closer to species may be tricky, since there are more than 40 species of mola skin parasites, according to http://www.centralcoastbiodiversity.org/ocean-sunfish-bull-mola-mola.html.
A bunch of these washed up on Ocean Beach.