Finding old candy can be a blessing and a curse. On the one hand, free candy. On the other, it's usually grown some weird white stuff in its five years under your bed, and you're not sure what the hell that is. Mold? Parasites? A ghost?? It's actually a "bloom" and it's totally safe to eat. We're going to tell you why.
As the educators at HowStuffWorks explain, there are two types of bloom: sugar and fat. Sugar bloom is usually dry and spotted, while fat bloom is streaky and greasier (because, you know, it's fat). Sugar bloom happens when the surface of the chocolate gets wet. The moisture dissolves the sugar, and, when the chocolate dries, sugar crystals remain on top. Usually, it's a storage problem -- as in, you're keeping the chocolate in a warm space, so its surface starts sweating.
Fat blooming happens when the fat in the chocolate's cocoa butter migrates to the surface. Hoarding your Snickers stash in a warm pantry can also kickstart this process, but rapidly changing temperatures are another trigger.