You can use half a cup of vinegar, rubbing alcohol, and baking soda.
You can also have a mixture of soapy water and baking soda.
A three-to-one mixture of water and bleach (the smaller portion) in a spray bottle is an excellent natural treatment for cleaning the coils on your air conditioner.
Bleach will not only get rid of mold and mildew but also keep them from coming back.