The answer to 'why is my window mildew' is that mildew is a type of fungus that thrives in moist, humid environments. Mildew can grow on windows due to condensation, poor ventilation, or water leaks. Symptoms include a white, gray, or black fuzzy growth on the glass or window frame.
To address window mildew, you'll need to identify and fix the underlying moisture issue. This may involve improving airflow, fixing any leaks, or using a dehumidifier. You can then clean the mildew using a diluted bleach solution or a commercial mildew remover. Be sure to wear gloves and work in a well-ventilated area. If the mildew problem persists or you're unable to identify the source, it's best to call a professional cleaning service.