You can spray disinfectant cleaner into the cowl to reduce that musty smell. Turn the air conditioner on, without engaging the recirculation feature, and spray the cleaner. Make sure there's nobody's in the car, because some cleaner will blow into the car. 

Check the underside of the car, as well. There's a drain under the evaporator that may be partly blocked. The air conditioner chills the air before blowing into the car, and when it's humid, moisture will accumulate. If your air conditioner running and you're sitting still for a long time, moisture will condense and fall to the ground. People who are not familiar with cars may think coolant is leaking.