It sounds like Chocolate bloom
My guess would be that when you observe this after cold storage, it is sugar bloom due to condensation cycling in your freezer & likely fat bloom when exposed to heat, though it could be both.
The chocolate should be salvageable by melting in a double boiler.
Further Explained from wikwpedia:
"Chocolate bloom is either of two types of whitish coating that can appear on the surface of chocolate: sugar bloom... and fat bloom"
"Sugar bloom is caused by moisture. Condensation on to the surface of the chocolate or moisture in the chocolate coating causes the sugar to absorb the moisture and dissolve. When the moisture evaporates, the sugar forms larger crystals, leaving a dusty layer. It is caused by:
- Storage of chocolates in damp conditions
- Use of hygroscopic ingredients (low grade or brown sugars)
- High-temperature storage conditions of chocolate-covered confectionery, where the centers have a high relative humidity and the water vapor given off is trapped in the wrappings
Fat bloom in chocolate is due to the cocoa butter which has separated toward the surface.2It is caused by:
- Poor (incorrect or incomplete) tempering of the chocolate
- Incorrect cooling methods, including covering cold centers
- Warm storage conditions
- The addition to chocolate of fats incompatible with cocoa butter