I have no idea what this is... Some kind of Platydoris or (remotely) possibly a juvenile Spanish Dancer?
The colours reminds me of some photos I have seen of Chromodoris alternata (an Australian species). Help anyone?
Found during a night dive at Ko Tachai, Similan Islands National Park, depth approx. 6m.

Best guess is Platydoris pulchra.
Cheers,
Marli
Best guess is Platydoris pulchra.
Thank you! 
Looks like Dr. Rudman agrees.....
http://www.seaslugforum.net/display.cfm?id=22283
Cheers,
Marli