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This specimen was identified by Dr. Richard C. Willan, thanks!
There are some pictures of very similar species from Anilao, Philippines online, which are labelled as Sclerodoris coriacea Eliot, 1904,
on the websites of Jeff Rosenfeld The Vibrant Sea: #1, #2, and NUDIPIXEL. S. coriacea is according to IPN known only from the Indian Ocean
of South Africa, Tanzania, and Madagascar, medslugs has a picture of a specimen from La Réunion Island, which looks similar to the specimen from Sodwana Bay, South Africa in IPN, page 187. On Philibert Bidgrain's website "South-west Indian Ocean Seaslug site" are some pictures of this species, also from La Réunion Island.
Most publications on the recent DNA based researches have shown that those species thought to have a wide Indo-Pacific distribution turned out to be distinct species or even a complex of species.
I guess Sclerodoris coriacea Eliot, 1904, is a grey-brownish species restricted to the Southwestern Indian Ocean and keep the orange species from the Philippines as Sclerodoris sp. 08 Genus: Eliot, 1904.
Gosliner, Terrence M., David W. Behrens & Ángel Valdés. 2008. Indo-Pacific Nudibranchs and Sea Slugs. Sea Challengers Natural History Books.
Gig Harbor, Washington. 426pp.