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The identification of this specimen was confirmed by David W. Behrens, thanks!
It was originally posted at these websites as Discodorididae sp. A Family: Bergh, 1891
Gosliner, Terrence M., David W. Behrens & Ángel Valdés. 2008. Indo-Pacific Nudibranchs and Sea Slugs. Sea Challengers Natural History Books.
Gig Harbor, Washington. 426pp.
IPN writes at page 171: "Known from tropical South Africa and the Kerama Islands of southern Japan."