May 15, 2006
From: Sayoko Matsuda
I always greatful for your advice.
I found this sea slug under a stone.
Locality: Otsuki-cyo Issai (the Pacific coasts of Japan), 13m, Kochi Prefecture Japan,, Pacific Ocean, 7 May 2006, coral and rubble site. Length: 5-6mm. Photographer: K's staff & Naoko.K.
Sayoko Matsuda, 2006 (May 15) Noumea hongkongiensis from Japan. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/16611
Your animal is N. hongkongiensis. It is a bit difficult to separate the two species you mention from photos, but the white colour of N. decussata is much more opaque than in N. hongkongiensis. You can see sinsome of your photos that the white colour on the mantle is almost translucent and you can see white spots around the edge which are the mantle gland. Another external difference is that N. decussata has fine yellow specks on the mantle, but these aren't visible in most photos.
It is good to get a record of this species outside of Hong Kong.