Certified GKH Gardening Expert
If they are chalky, it could be the type of water your are using. It may be mineral deposits building up on the leaves. Watering with a distilled water will stop this.
If they are fuzzy, this is mealybug and this article will help you treat it, as will neem oil, which is safe and effective: https://www.gardeningknowhow.com/plant-problems/pests/mealybugs-white-residue-on-plants-leaves.htm
If it's more powdery in nature, then powdery mildew may be the problem. Neem oil will help with this too. Here is more info: https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm