It is actually the Glaucus atlanticus, a nudibranch that only eats the dangerously venomous Portuguese Man o' War Physalia physalis; the by-the-wind-sailor Velella velella; the blue button Porpita porpita; and the violet snail, Janthina janthina. they are poisionous, but very beautiful. Nudis are very unique, they only eat specific foods, and without those specific foods they starve to death, they will not eat anything but their specialized prey.
I dont do much with online coral vendors, all my corals have come from local clubs. Cherry has some real fine corals, as does AquaSD, and Sexy Corals. All the corals you see on line are usually photos under heavy Actinic lights (actinic is 400-480 nm) cause that is the best PAR for the corals, and really makes it pop. a plain green coral will look dull under regular light (10000k+) but glow like radioactive waste under the right light.