Most chocolate mnfctrs guarantee "chocolate", that is, the real stuff with out any butterfat in it, for 2 years. Chocolate has cooca fat,but not butterfat,which comes from milk powder. Since chocolate has no water, I repeat, NO WATER in it, it doesn't go "bad" but will go stale if allowed to. I've had and used chocolate over 5 years old, stored under decent condtions with no flavour or working compromises.
Milk chocolate, on the other hand, contains about 1/3 milk powder, which will go eventually rancid, and white chocolate, contains over 40% milk powder, so it goes rancid even faster. Most mnfctrs, guarantee milk choc at 9 mths, and white at 6 mths.
"White stuff" will apppear on the surface of chocolate for two reasons,
The first is "fat bloom", where the chocolate was kept at temperatures over 30 C (90 something? F) The cocoa butter is very temperature sensitive,and at temps over 30 will melt and recrystalize at unstable forms--gving you a white chalky/moldy appearance. This is by no means bad, it's perfectly fine to use or eat, but has a better "mouthfeel" if remelted and re-tempered
The second is "sugar bloom" and looks almost like fat bloom. Sugar bloom indicates that moisture got into the chocolate, most likely from being stored in the fridge or freezer. I told you that chocolate has no water in it, right? But it has sugar in it. When chocolate is stored in the fridge and the taken out, the surface gets wet. Condensation,. Same as morning dew on the grass. This moisture dissolves the sugar in the chocolate,and when it dries, the sugar recrystalizes, giving you a chalky white surface. Again, not bad, perfectly safe to eat, just looks gross.
Hope this helps