Yep, this greyish slug looks rather drab — and indeed it is often overlooked, even when found in large numbers. It is “common” throughout the world and at times can be utterly prolific in certain areas.  Apparently, it can take on darker colours, and even a reddish colour, depending on its food source — which is always some species of anemone.  Look closely to make sure you have not found an Aeolidia loui, a sea slug that looks very similar (but has noticeable bumps on its rhinophores).  The loui also has  some very cool super hero attributes.  Wouldn’t be surprised if this one does too.  Usually about 2″ or 50 mm, but some have noted individuals as large as 4″ or 100 mm.