Could be dissolved solids from water or more likely some pest eggs. You can use diluted castile soap with a bit of cayenne pepper mixed in as a spray treatment for a lot of pests like these. Works well, is cheap plus it’s all natural so its as safe as can be.

Is it in a humid area with low air flow? Powdery Mildew is tough to get rid of and very easy to spread. You can try diluted hydrogen peroxide spray, neem oil, fungicide, etc but getting rid of Powdery Mildew is an uphill battle.

Early powdery mildew?