Mold in closets usually occur on the closet’s outside wall of the house and appears as dark stains near the floor. An outside wall can get very cold. When you open the closet, warm moist air from the house flows into the closet and condenses on the cold wall. You remove your clothing, close the door and that moisture remains on the wall with little to no ventilation to evaporate it away (also see Mold in house).
Dark damp surfaces are a perfect medium for growing mold; hence, your dark stains on the base of the wall. Enough surface area of mold can become a health hazard.
To prevent mold you have to remove the moisture, or at least let it vent away when it accumulates.
To vent the closet we need a vent at the top and the bottom of the area. Two vents will allow air to circulate, evaporating any accumulation of moisture.
The vent at the bottom is easy; just make sure the door is cut with a 1 1/2″ gap at the bottom. The upper vent could be in the door or in the foot of plaster or drywall above the door. Use a 10″ X 6″ flat heat register for the upper vent trim. You can cut the hole 10″ wide by 6″ tall because a 10 X 6 register measures 11 5/8″ by 7 1/2″. You can mess up the hole a little bit and still hide the cut marks.
From inside the closet check to see if there is a solid wood header over the door. Take a small drill and punch a few holes across the area above the door. If there is a header over the door, put the vent in an adjacent wall of the closet or use the door.
Attach the vent covers, one in and one out, with molly anchors in the plaster or drywall. Install the outside vent with the louvers aiming up so you cannot see into the vented area. This is simply for esthetics, not better airflow.
Since you emptied out the closet for this project you can clean up the stains.
Sponge a solution of 1 part bleach to 4 parts water on any stains or discoloration in the closet. Do not get the wall dripping wet, just damp. Leave the bleach on for ten-minutes or so. Wipe off all the walls with clean warm water and leave the door open until it is dry.
Finish this job with a coat of mildew-proof paint to provide a surface resistant to further mold growth.
Your closet will shine like a star and smell fresh as a daisy for many years to come.
You can also insulate the wall to warm it up and prevent the condensation, but the cleaning and painting should still be done to remove any mold.