This is one of the very many small white dorids found throughout the Salish Sea/Pacific Northwest.  Fortunately it is a little easier to identify than some of the others. There is usually a brightish white bump behind its branchial gills, its mantle is covered in tubercles that vary from large white bumps to club-like and it often has a brownish tinge to the centre of its mantle.  While it is more common to find them on kelp feeding on kelp bryozoans, I have found them on rocks in caves and on barnacle covered ledges.  Tiny — to about 20 mm or 3/4″.