What is the white stuff on Brie Cheese?

Actually there's more to moulds than meets the eye (excuse the pun!). I'm a Microbiologist and can confirm that you get good moulds and bad moulds. Moulds by the way are actually called fungi. Bad moulds: athlete's foot, bread mould, musty ceilings, potato rot, etc. Good moulds: Penicillium (one species actually gives us the first antibiotic - penicillin), edible mushrooms (you get deadly poisonous ones too), and ones you use for cheese making (healthy) - now if you leave the cheese outside for a long time it will get another mould, normally Aspergillus, which spores float around in the air at any given time. Have you seen blue-veined cheese? That's actually the hyphae (branches and roots) of a mould. Sorry for the science jargon, I couldn't resist!