Buried under mud in eelgrass bed.
Possibly an undescribed species rather than S. enosimensis: http://slugsite.us/bow2007/nudwk1236.htm
possibly?
As seen in the 3rd image, the shell was inhabited by a hermit crab.
Low tide -.82 rocky intertidal.
Found by @luanroberts. About 7 mm. Orange bands on white-tipped oral tentacles, orange bands on the white-tipped rhinophores, and white specks on the surface of the body.
Obelia and Tubularia (Pink-mouth hydroid) present elsewhere in the marina in small quantities, but didn't see it feeding.
Looks vaguely like the pale one in the left-side photo on Page 124 of Behrens & Hermosillo's Eastern Pacific Nudibranchs, 2005. Same individual as https://www.inaturalist.org/observations/91332884
Update: this hydroid https://www.inaturalist.org/observations/92299348 was the only hydroid on the boat bumper where it was found.
Found washed up on beach. Very slimy like a slug. About 2" long.
Oh man. I cannot id this one. It seems special though. I noticed it predawn. It was moving. As soon as my light shined on it, it stopped, sank and became flat.
*Siphon of a boring clam
*I'm referring to the specimen on the left.