This unusual Velutinidae is extraordinarily difficult to see as it is small (less than 2 cm) and it looks like the white ascidian sponge or bryzoan it usually feeds on.  The first time I noticed one I thought I had somehow knocked off a  barnacle, but it continued to move quickly across the substrate and extended its head tentacles.  While it looks like a sea slug, it is actually a sea snail and a member of the sub-family lamellariinae . The fact that its mantle covers its shell makes the original identification difficult. Bill Rudham, of Sea Slug Forum fame,  offers a good explanation of what qualifies a lamellarlinae/Velutinidae as a snail. I have yet to see others in this family in the Salish Sea, but I have come across a few in Hawaii.