The white film on chocolate is called “bloom” and is caused by excessive or varying temperatures. You have committed the crime of chocolate abuse by not storing it properly.
The white film is not mold and is perfectly harmless, affecting only the chocolate’s appearance and to some extent its texture. Milk chocolate typically consists of about 70 percent powdered milk and sugar, with only about 12 percent chocolate liquor, so it isn’t as prone to bloom as is dark chocolate, which may contain as much as 75 percent.
There are three kinds of chocolate bloom: fat bloom, sugar bloom, and age bloom.
Fat bloom happens when under conditions of excessive warmth some of the liquid fat constituents migrate to the surface, where they form relatively large, light-reflecting crystals. Chocolate should never be stored at a temperature higher than 80°F (27 °C); 63°F (17°C) is ideal.
Sugar bloom happens when the chocolate is wet or stored in high humidity, which dissolves some of the sugar out of the surface, where it remains as solid crystals when the water evaporates.
Age bloom happens to old chocolate, when the slow-forming fat crystals of form VI have had a chance to develop. They form big, coarse crystals that disrupt the smooth texture of the chocolate to the extent that it may actually crumble.
For example, if you drop a bar of chocolate behind the seat of your car and forget about it, discovering it only two years later when you’re cleaning out the car before selling it, you will notice, oh, I can’t go on. It’s just too horrible to contemplate.
The best way to avoid chocolate bloom is to consume all chocolate as soon as it comes within reach. That’s my method.