2009 Chevrolet Impala moldy smell from vents

Visit the dealer's parts department. They have a spray bottle of odor neutralizer.

Look under the hood on the passenger side of the firewall for a 4" long rubber hose with a 90 degree bend hanging down. That's the drain hose for the water that condenses on the AC evaporator. Squeeze the end of that hose to see if any debris can be dislodged. You can squeeze the wire ring around the end of the hose and remove it too to check inside further for blockage. When that hose becomes blocked, water can build up until it sloshes onto the floor when you go around a corner. Before it gets that bad, you can get that musty smell if the water doesn't all drip out onto the ground.