Nembrotha cristata  Bergh, 1877

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Nembrotha cristata is similar in size and colour to Nembrotha kubaryana, its main point of difference is the green border to its foot. It feeds on the green compound ascidian Eudistoma olivaceum. This shot shows a pretty pale colour variant, perhaps a different species.
More information on Nembrotha cristata are on Bill Rudman's Sea-Slug Forum!