oh - it’s an arminid; Ornate Dermatobranchus
OK… got the name (from the last trip review, Similan liveaboard), it’s actually an Arminid (one of the 4 distinguishable categories of nudibranch - Arminids, Dorids, Dendronotids and Aeolids), Ornate Dermatobranchus - normally found on sandy bottom, feed on soft coral, hydroid.
There were 2 when we spotted them. Size to 8cm (about, things enlarged underwater). Photo taken by Larry, one of the ‘holidaying’ dive master on board.
The last time we saw a Dermatobranchus gonatophora in Rechellieu, think Similan area have some good variety of Arminid 
