This is called the "bloom." When chocolate is stored at
temperatures above 78 degrees F. it melts, causing the cocoa
butter to rise to the surface. This is called "cocoa butter
bloom." When condensation causes sugar to dissolve and rise to
the surface, it is called "sugar bloom."
Bloom does not affect the flavor or quality of chocolate and
when used in a recipe, it will regain its original color. To
prevent bloom, store chocolate tightly coverd in a cool, dry
place.