The air smells musty on Window Air-Conditioner unit ?

Air conditioners remove moisture from the air. The water collects in the base of the unit. Under normal conditions this water will be evaporated out of the unit. However, it is possible for some water to sit stagnant in the base of the air conditioner for extended periods of time. There is no easy way to prevent this problem. The problem will be reduced if you carefully clean the base of the inside of the air conditioner at least once a year. That will keep any dirt, lint or dust from absorbing the water and allowing mold and mildew to grow. Also, replace the filter behind or in the front cover.