The bloom (as this discoloration is called) is almost always caused by improper storage and is usually one of two substances. It could be cocoa fat that rose to the surface of a solid chocolate mixture that was stored too long at room temperature. Or it could be sugar that was drawn to the surface by the condensation of a loosely wrapped, refrigerated chocolate mixture; the moisture extracts sugar from the mixture's interior. In either case, the bloom is unaesthetic but harmless.
** Asian Recipes