White Dust: Is it bad for electronic devices like smart TVs or phones?
White dust:
https://www.hvac.com/faq/white-dust-concerned/  


  White dust is usually caused by mineral content in the water that goes into a humidifier. When the mist lands on furniture or other surfaces and dries, it can sometimes leave behind the dried mineral deposits, or “white dust”.


The dust is a salt (ionic compound).
The dust should be safe when dry, but as soon as it absorbs moisture it will become electrically conductive.

I would be more concerned about the liquid water that appears before the dust.

You could use distilled water as suggested by @jsotola to avoid the issue altogether.



I was wondering for myself a moment why "normal humidity" doesn't cause the white dust, but only the humidifiers do:

Ultrasonic humidifiers work by splashing water everywhere, the same thing you can do with your arms in water, the only difference being the size of the droplets.

So you should definitively use distilled water as the humidity from an ultrasonic humidifier is basically like splashing water on electrical appliances.

Tiny droplets of distilled water shouldn't be much different from normal humidity which generally isn't an issue.