Phyllidia coelestis  Bergh, 1905

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Phyllidia coelestis is characterized by the black "Y" on its head, by the yellow-capped mid-dorsal tubercles never forming a median ridge, and by the broad blue-grey mantle margin. Phyllidia coelestis has no dark stripe on the uniformly blue-grey foot sole.
Pretty similar is Phyllidia varicosa which has tubercles forming many longitudinal ridges, yellow-capped tubercles at the mantle margin and a dark longitudinal stripe on the blue-grey foot sole.
More information on Phyllidia coelestis are on Bill Rudman's Sea-Slug Forum!