Faeologists used to think that the Moodzies were made up of multiple "species" of Faerie that fed on different emotions, feeding back the emotion into the human until the biofeedback satiated the Faerie or killed the human. But there is a new theory that perhaps the Moodzies are one single type of Faerie that begin as a white or grey, but when an emotion comes by, they respond to that emotion and begin to morph into the Moodzie suited for that particular emotion. Once done, they slowly fade back to their lethargic, pale, original forms.
(I really, really want to dive into the Faerie realm one of these days. Just inventing my own versions as well as rehashing some old ones. I think some reading is in order.... oh and another clone so I'll have more time to do it.)