Looks like you may have thrips.
Captain Jacks dead bug applied every couple of days will help eradicate the buggers.
Take a good look, are those pests. Look like thrips
Does it wipe off? If not thrips it could be your humidifier. If it is thrips, I second Captain Jacks.
Do you use an ultrasonic humidifiers or cool mist impeller humidifier? Do you use tap water in a humidifier?
The only types of humidifiers that can potentially release white dust are ultrasonic humidifiers and cool mist impeller humidifiers
White dust is a byproduct of minerals found naturally in water, which can sometimes be released into the air during the humidification process. White dust, which can be a nuisance but is not harmful, may settle on surfaces.