Whitish powder coating on my wheat leaves like flour, is it harmful?

A white to grey powdery coating that you can wipe off the leaves is powdery mildew, a fungal disease that prefers cool, humid, cloudy weather and thick, shady, heavily fertilised crops. It usually starts on the lower leaves and can spread up to the flag leaf, reducing the green area that fills the grain if it gets bad. Please confirm with a photo to your KVK or agri officer. Cultural steps that help are avoiding excess nitrogen and very dense sowing so the canopy is not so humid, and choosing tolerant varieties where mildew is a regular problem in your hill area. If it is moving up to the upper leaves and the crop is still some weeks from harvest, recommended fungicides include actives such as propiconazole, but follow the label dose, the pre-harvest interval and your local KVK recommendation and rotate the mode of action. Catching it while it is still on the lower leaves keeps the upper leaves clean for grain filling.