It looked like a drab piece of slime on the sandy-muddy area transiting between coral rubble and coral reef. But upon closer examination, it really is a slug, with some really gorgeous blue dots.
Look at that blue... This species has been spotted on the island before, and is listed on Semakau's mollusc list recently published in NIS. The Sea Slug Forum shows a whole array of colour variations of this species (which may very well be more than one species). The site also reports that this saccoglossan "stores huge numbers of bright green chloroplasts in ridges hidden from view beneath the parapodial flaps". It is hypothesized that the chloroplasts are deliberately shaded from the bright tropical sun as most chloroplasts have an optimal light intensity at which they photosynthesise and "can be burnt out if exposed to too much light".
Cool stuff. Should have opened the flaps. :)