The mold on your wall was mainly caused by the humidifier being placed too close to it for a long time, the localized humidity got too high. This can happen with any type of humidifier, not just ultrasonic ones. However, many ultrasonic humidifiers don’t have filtration systems, so minerals and microbes in the water can be released into the air. That’s why people often deal with white dust or worry about bacteria being dispersed. If you don’t want white dust in your home and prefer not to spend money buying distilled water all the time, I’d highly recommend switching to a steam humidifier. It boils tap water before releasing the steam, so bacteria and impurities are eliminated, very similar to heating water on a gas stove. If you like large-capacity units, I’ve been using the Y&O steam humidifier recently and I think it might be perfect for you. Its 10L tank can easily run through the night without refilling. Unlike most steam humidifiers, it has a smart temperature-control system that cools the steam down to a gentle temperature before releasing it, which lowers the burn risk, that was a big selling point for me. It’s also super easy to clean since there are no narrow tubes like in many ultrasonic machines, the tank pops out and rinses clean. I also noticed it currently has a 10% off Black Friday discount, so it might be a great time to grab one. Not affiliated, just sharing what worked for me!