Then it is probably a difference in ultrasonic humidifiers. I can tell you that mine barely builds up much slag at all. I switched to warm-air after I grew tired of the white-dust issue.

The white-dust issue is further confirmed as I use the warm-air (boiling-type) humidifier. A LOT of minerals are found every day from my boiled-away tap-water... minerals that clearly used to be sprayed into the air and dusting my furniture.