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"I can't put a name on it right off. It has some similarities to Elysia japonica - the dark tips of tail and rhinophores, but it is a lot paler than any I have seen before. I have seen some very pale ones, but usually the inside of the parapodia would show the contents of the digestive gland tubules through the skin. This is usually difficult to see in the field becase they keep the parapodia closed. Elysia japonica also has a characteristic constriction of the pericardium, which I cannot see on your pics."
So I call it Elysia cf. japonica - cf. is an abbreviation from a Latin word (confer) and means 'refer to' or 'compare with', here it means it could possibly be Elysia japonica or perhaps something closely related, or at least similar in appearance.