In the waters of the Salish Sea, this headshield slug seems to be a little less common than the Blister Glassy Bubble. Upon initial sighting it can appear very similar to the darker version of the Hamaena vesicula, but its shell is usually covered by large outer flaps, and its head has very distiguishable white marks.  For a great photo of this animal loosening its flaps as it is caught in the tide see Dave Behrens’ Slugsite.  Behrens is an expert on Opisthobranchs. Another useful source is the Sea Slug Forum.  I’m not sure if these experts know any more about feeding patterns than they did ten years ago, but the best guess then was marine flat worms.