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It was originally posted at these websites as Nembrotha kubaryana which is still the most common name of it.
Let me quote
Pola, M., Cervera, J.L. and Gosliner, T.M. 2008. Revision of the Indo-Pacific genus Nembrotha (Nudibranchia: Dorididae: Polyceridae), with description of two new species. Scientia Marina 72(1): 145-183 :
"Bergh described Nembrotha nigerrima in 1877 from two different specimens collected at Palau and Cebu Island. In the same paper, Bergh described N. kubaryana also from Palau" ..."N. kubaryana is a junior synonym of N. nigerrima. Similar conclusions have already been presented by Yonow and Hayward (1991), who formally synonymised both names."
Yonow, N. and P.J. Hayward. – 1991. Opisthobanches de I’Ile Maurice, avez la description de deux espèces nouvelles (Mollusca: Opisthobranchia).
Revue fr. Aquariol., 18: 1-30.
On 08 June 2011 changed into Nembrotha nigerrima Bergh, 1877 because of the above text of Pola et al.,2008,
On 11 January 2012 changed back into Nembrotha kubaryana Bergh, 1877
because Nathalie Yonow points out in her recent publication that Yonow and Hayward (1991) had selected Nembrotha kubaryana as the valid name:
Yonow, N. Results of the Rumphius Biohistorical Expedition to Ambon (1990). Part 15. The suborder. Doridina (Mollusca, Gastropoda, Opisthobranchia, Nudibranchia). Zool. Med. Leiden 85 (17), 30.xi.2011: 905-956, figs 1-22, Plates 1-8.— ISSN 0024-0672.
Pretty similar in size and colour are:
Nembrotha cristata, which lacks the red border to its foot and
Nembrotha livingstonei which differs by a distinctive white cross-like mark between the rhinophores
More information on Nembrotha kubaryana are on Bill Rudman's Sea-Slug Forum!