Video: https://flic.kr/p/W9aHNg

About 2/3" long, smaller (and cleaner) than most in other photos on the internet.

UPDATE 6/29/17: From my Flikr post of the same photos, Jeff Goddard ID'd this as Thordisa bimaculata and provided an excellent explanation.

Email from Dave Behrens 6/26/17: "Yep - sure looks like it. Never seen an intertidal specimen." Thanks - Dave Behrens