This is a species of sea slug called Philinopsis cyanea. Found in areas with sediment and rough sand, its main prey are small shellfish which it locates by following the slime trail.
Quite a few species exist here in Laamu Lagoon- this is the most colourful one that I have found thus far.
Apparently the slug below (photographed on the same dive) is the same species but with a different colour variation.
Finally (and also on the same dive) is a head-shield slug. This is the first variation of this I have seen. Although small apparently they rip in to their prey and are voracious carnivores.
Finally the last photo (also same dive) is some kind of Headshield Slug. This is the first time I was able to get a photo of this mini-terrorist. They are also carnivores and do unspeakable acts with their digestive system.