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Pretty similar are:
Phyllidia coelestis which has a "Y" shape of the blue-grey dorsal ridges and yellow-capped mid-dorsal tubercles never forming a median ridge and a broad blue-grey mantle margin. Phyllidia coelestis has no dark stripe on the uniformly blue-grey foot sole.
Phyllidia carlsonhoffi which has alternating large conical and small rounded tubercles (never joined as ridges), and also a median black line on the foot sole.
More information on Phyllidia varicosa are on Bill Rudman's Sea-Slug Forum!