I think this is a Cumanotus, but not sure of the species - the green criss-cross marks on the back half of its body do not appear in other photos of C. fernaldi, and the pink tips on the cerata also appear to be different. Also, the overall body color appeared more bluish-green in sunlight, not cream as these photos show. Body was about 1/2" long. Gorgeous!

UPDATE: An undescribed species (so far) - as per Gary's notes: http://www.inaturalist.org/observations/2016509 I'm unreasonably proud to have 'discovered' this large colony of them!