As a side result of using an ultrasonic air humidifier, this white dust thing can be seen on house furniture during its activity. I googled about it but I only found a few articles, saying that it is safe for human body but I found nothing about house devices. Does it have any bad effect on my LED TV? I use a homemade cover for it but, I'm still kind of worried.
Ultrasonic humidifiers have a way of vaporizing that causes the lime sediment in water to evaporate and condense on any nearby surface unlike steamer and spin drum (hot & cold types) .
A demineralization water filter can work.
Lime can be harmful , long term , to lead solder joints in copper plumbing, so water softening is recommended to remove the minerals. Boiling water will also leave a white sediment in kettles, coffee boilers and cooking pots, according to how “hard” the water is.
Lime is not conductive or very salty as far as ionic levels are concerned so it will be harmless when dry.
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.