This nudibranch, upon first glance, looks pretty much identical to another, the Aeolidia papillosa.  If I hadn’t thrown the image up on inaturalist.org, I would still be thinking this individual was a Common Grey and not a Shaggy-mouse.   The difference lies in the tiny bumps found on the rhinophores of the Aeolidia loui.  So look closely, very closely.  These animals can often be found under rocks curled up in a ball on the anemones that feed them.  This particular individual was found in about 6 inches of water.   Common it is not (at least in the sense of being usual).  This nudibranch has some super hero qualities.  It is  able to live symbiotically with the anemone upon which it feeds and, through photosynthesis, is able to act as a mini carbon sink . It also has the ability to chemically manipulate its prey and keep it from discharging its nematocysts.   See inaturalist.org for further, more scientific discussion of these abilities.