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Pretty similar is
Costasiella cf. kuroshimae, which differs by the colour of the head, white in Costasiella kuroshimae, by the broad brownish band on the head, in Costasiella kuroshimae a thin line, and by the rhinophores: lower half white and the upper half black.
In IPN are pictures of two different species as Costasiella kuroshimae, the left one is my Costasiella cf. kuroshimae, the upper right one might be Costasiella kuroshimae, its rhinophores are white with a black tip.
More information on Costasiella kuroshimae are on Bill Rudman's Sea-Slug Forum!