In case you observe mildew around the window sill or body, handle it 1st. Protect the wall and flooring under the window which has a plastic drop cloth. You could cleanse nonporous surfaces with cleaning solutions, gentle dish detergent and water or a solution of water and bleach based on https://dailyspecter.com/achieve-spotless-surfaces-with-pure-water-cleaning-the-ultimate-solution-for-window-and-building-cleaning/