When the top or sides of the wax has a crystalline, white-ish color coating, we refer to this as frosting or blooming. Frosting is a natural characteristic of soy and it is one way that a customer can tell if their candle is really made with pure, all-natural soy wax. Frosting does not affect the scent throw or the burning properties of the candle - it is just a particular crystal growth that can form on the top and sides of the candle.
To minimize frosting, try to keep your candle out of direct sunlight and fluorescent lighting. Changes in temperature when the candle is being cooled can also cause frosting. While these tips may help, it is almost impossible to completely control frosting - it is just one of the beautiful and natural features of this wonderful wax.