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It is most probably Polycera sp. 3 in IPN at page 103:
Gosliner, Terrence M., David W. Behrens & Ángel Valdés. 2008. Indo-Pacific Nudibranchs and Sea Slugs. Sea Challengers Natural History Books.
Gig Harbor, Washington. 426pp.,
Polycera sp. 2 at Jun Imamoto's website umiushi - Sea Slugs from Japan and
Polycera sp. #1 at Sea Slugs of Hawaii