This is a cute dorid, one that is a joy to watch as it moves across its sponge prey.  Apparently, northern animals (such as those found in the Salish Sea) differ significantly from their southern counterparts in that they have numerous brown spots/circles on their bodies and they can appear to have slightly darker mantles.  Spotted Leopard Dorids are known to grow to 10 cm or 4 inches and are easily spotted, though not that commonly seen by snorkelers or free divers.  The first image in this gallery is of a juvenile.  It was no larger than .5″ or 12 mm and there were at least five others in the area.  Check out the Sea Slug Forum for more discussion on their colour differentiation and taxonomic history.