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Maybe this is a strange variant of Trapania japonica.
This species is not in IPN, there are pictures of a very similar, most probably the same species from Flores, Indonesia at the Sea Slug Forum, and as 'Opisthobranch of the Week # 762" at Michael D. Miller's The Slug Site,
Nudi Pixel has pictures from Anilao, from Taiwan, and one more from Taiwan.
The species which was identified by Dr. Juan Lucas Cervera Currado as Trapania japonica is at Nudi Pixel as Trapania sp. with a picture from Koh Rong, Cambodia,
the other etreme variant, a specimen almost without brown patches is as Trapania sp. 8 at www.umiushi-zukan.com.
There is either one very variable species or a complex of pretty similar species, with a white body colour, yellow pigment on the processes and dark brown pigment on the anterior face of the translucent whitish gills.
Pretty similar is also:
Trapania naeva, which differs by dark brown/black lateral processes and rhinophores.