I found this there under "About Chocolate"...>
Why does the chocolate sometimes get a white layer on top?
When a thin layer of fat crystals forms on the surface of the chocolate, it is called fatbloom. It means the chocolate has lost its gloss and a soft white layer appears, giving the finished article an unappetizing look. Fatbloom is caused by the recrystallization of the fats and/or a migration of a filling fat to the chocolate layer. Storage at a constant temperature will delay the appearance of fatbloom.