I wrote to the Sea Slug Forum:

"Looking for an id on this guy. About the closest I can find is Diaulula sandiegensis, and Dave Behrens notes that the spots can be different or missing."

Behrens replies: "Actually this is the white variation of Doris montereyensis (previously Archidoris montereyensis). This white variation always confuses folks, so don't sweat it. Diaulula has a velvety looking dorsal surface due to the spicules protruding from tiny tubercles. Doris has much larger, rounded tubercles like in your photo. Just like the common yellow form of this species, the black spots extend over some of the tubercles."

Best wishes,
Dave Behrens
Behrens, D.W., 2006 (Mar 27). Comment on Diaulula sandiegensis? from Puget Sound by Jack Connick. [Message in] Sea Slug Forum. Australian Museum, Sydney. Locality: Rosario Beach - Urchin Rocks, Deception Pass State Park, 30', Washington, USA, Rosario Strait, Puget sound, Pacific ocean, 18 March 2006, Rocky crevice. Length: 3.5". Photographer: Jack Connick.