It’s mineral deposits from the water your misting with. Mist with distilled water, though I should add misting doesn’t really help so you don’t have to if you don’t want to. Cool mist humidifier with distilled/RO water would be a much better solution to raise RH.
As an fyi, if you put tap water in a cool mist humidifier, the minerals will slowly leave a white oily film on everything in your house.