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It is Trapania sp. 3 in IPN:
Gosliner, Terrence M., David W. Behrens & Ángel Valdés. 2008. Indo-Pacific Nudibranchs and Sea Slugs. Sea Challengers Natural History Books.
Gig Harbor, Washington. 426pp. on page 137
Pretty similar are:
Trapania toddi which has a white background colour with reticulate brown patches in which are small white spots.
Trapania euryeia which has translucent clear rhinophores with varying amounts of brown, white and sometimes yellowish colouration.
It was originally posted at these websites as Trapania sp. B Genus: Pruvot-Fol, 1931,
for some genera, such as Trinchesia was the alphabet almost through. I had to change the distinguishing alphabetic character into a number. I have chosen 2-digit numbers, even though '01' looks a little bit stupid, and now is only the genus italic written.