Inside or outside?

Inside is the evaporated oil from the dash. Either from the plastic or from the greasy dash protectant you are using. There are some light degreasers you can use on the inside. Then use a glass cleaner.

Outside: prolly calcium residue. Using a glash polish can help remove this calcium layer.

Use drinking water in your washerfluid reservoir. Using your washer fluid lessens calcificaton.