Why does chocolate sometimes develop white or yellowish-white patches on its surface?

The condition is known as  sugar or fat bloom, which can occur when chocolate undergoes condensation as a result of temperature changes.  When temperatures change, sugar separates and rises to the chocolate's surface, leaving white swirls or spots. You may have noticed this if you've stored chocolate in your refrigerator. When chocolate melts and then resets, fat bloom reflects that change in cocoa butter structure. The result is yellowish-white streaks in the chocolate. These streaks do not indicate that the chocolate is unhealthy to eat or will taste bad.
 
Source(s):
Chocolatecandymall.com "Simple Secrets for Storing Candy"