No, that's not mold. That's cocoa butter.

Cocoa butter is a very interesting kind of fat – one of the few vegetable fats that is solid at room temperature. And when we melt it and let it re-solidify properly it goes through a process not unlike what needs to be done with tempered glass and steel, called tempering.

Chocolate has three main ingredients: cacao solids, cacao butter and sugar. Cocoa solids make up the dark part, sugar is sugar, and cocoa butter gives it its texture, shine and consistency. Cacao butter displays a kind of polymorphism, which means that it contains certain types of crystals that form chains that give chocolate its solidness. I won't get into too many details, but there are four types of crystals and one of them, ß crystals, gives chocolate the desired appearance and snap.

When you melt chocolate, you break up these chains and the crystals get dispersed. As the chocolate cools, these different kinds of crystals form their chains. Since crystallization is a chain reaction, it's only a matter of time before everything turns solid again.

There is a heating-cooling curve that needs to be followed according to each type of chocolate to make sure you get ß crystals and the desired consistency. If tempering is not done right, you get other types of crystals and an uneven texture, with cacao butter "blooming" out of the chocolate, and the chocolate becomes brittle, dry and opaque. That's what happens when a bar of chocolate is left out in the sun, re-solidifies and looks all white and weird.

So that's what happened to your chocolate. You left it out to cool by itself, without tempering it, and the "wrong" types of crystals formed. So the white stains are nothing but cocoa butter. If you re-melt that chocolate and temper it properly, you can have beautiful, shiny chocolate again.

Also, chocolate can't really mold, because it's anhydrous – does not contain water. Mold needs water to grow.