If the water you are using has a higher than average mineral content or the water is extremely hard, it can cause the ultrasonic humidifier to disperse the minerals as a white dust. This happens because humidifiers work through a steaming process whereby water is heated up and turns to a vapor before being sprayed out. This white dust is in no way dangerous or harmful to human health and is a natural by-product of the water.
To resolve the issue you can use distilled water in the humidifier.