Ultrasonic Humidifiers x 2

These foggers have a ceramic disc which vibrates at very high frequency.
The `sound waves’  break the liquid water into airborne fog particles.
Water, if not close to pH neutral, (and pH is a log scale – E.g. 8.0 is 100 times more alkali than 7.0) will throw a cast - calcium if the water is too alkali, or rust if too acid.
The ceramic disc rim becomes fouled and the disc starts to vibrate off-frequency then stops.
We say on the label of our machines and in the book to monitor our water and treat it if it’s excessively off-basic. Tank rain-water contains whatever dust settles on our roofs. Town water is treated only for contaminants not for its pH. As such there is NO WARRANTY on these foggers. And of course, yes, you may have perfect water quality, used distilled water even, but a puck is an inexpensive consumable item with an uncertain service life.  

Try first soaking it in a CLR solution or vinegar, and a soft toothbrush to clear the fouling.