Via: iNaturalist: Spanish Shawl (Nudibranchs and Other Sea Slugs of California)
Wikipedia: Flabellinopsis iodinea
Flabellinopsis iodinea, the Spanish shawl, is a species of aeolid nudibranch, a very colorful sea slug. This is a marine gastropod mollusk in the family Flabellinidae.
This species is native to the west coast of North America and farther south. It has been reported as far north as British Columbia, Canada, and as far south as Punta Asunción, Baja California Sur, Mexico. In addition it is found in the Gulf of California and the Galapagos Islands.
It has been found off the coast of Santa Catalina Island, California. It has also been found off the coast of the Palos Verdes Peninsula and San Diego, California .
Fire Slug, Fire Slug!