Cleaning spots on windshield.
I work in the glass industry. Windshield delamination does not look like spots, but always starts from an edge. Often it will have an oil slick look where it is delaminating.
Glass is hard to spot or stain, but it is possible. Most likely source is out gassing from vinyl dressing used on the dash. If you could feel feel the stain a razor blade or very fine steel wool often will clean it off with no glass damage. Try a cleaner high in content of IPA alchohol, like Perfect Glass.
If you have a stain there is a product called Glass Wax available at some auto, hardware, or department stores. Glass Wax can be used to buff out the stain. Rain X used on the surface with stains also can help, but I think it just covers up the stain. Either of the above may take multiple applications. Glass shops have special compounds and buffers that could buff out the stain. Good luck.