This is one of the largest nudibranchs on the West Coast, getting up to 20 cm or 8″.  They are rarely seen by snorkelers or tide-poolers as they prefer the subtidal zone.  This individual was found approximately 10 ft below the lowest tide mark and was at least 7″ or 17.5 cm. They are usually pure white (almost looking bleached) and have no brown spots on their dorsum, mantles, or feet.  For more information check out the Central Coast Biodiversity site, which also takes you to links on inaturalist.org.  For those inclined towards engaging in citizen science and documenting the bio-diversity of the Pacific Northwest/Salish Sea, inaturalist is definitely worth checking out on all levels.