It's hard to tell from the image. It could be damage to the anti-reflective coating or it could be trapped humidity (it's autumn and temps dropped significantly, the towel may have obstructed airflow and the trapped air in the scope may have reached dew-point). Quick check: Press (gently) with your fingers on the screen (several points). If the "stains" stay the same, position, form and size, it is likely damage to the anti-reflectice coating. If they change it's likely trapped humidity or delamination of the display stack.