Interesting nudi found under a rock. I think it was on some kind of sponge, which I foolishly failed to photogrtaph. Pretty sure it's Goniodoris. Maybe Goniodoris meracula?
This species of pleurobranch varies widely in color: from pale, nearly white, to a deep brown that's almost black (as in this observation). B. martensi can autotomize parts of its mantle when threatened. It feeds on sponges.