Plaster walls can absorb a great amount of water if exposed to the right conditions. If the water is allowed to sit on the wall, mold may appear. Mold is a fungus that will spread quickly. If it is not killed, mold may cause respiratory problems as well as the exacerbation of allergies. Cleaning the wall will temporarily remove the mold but it is a good idea to improve air circulation in the air with a fan, or install a dehumidifier.
Put on your safety glasses, breathing mask and gloves to protect you while cleaning.
Mix together a non-ammonia based soap in water and make suds. Dip a stiff bristle brush into the water and scrub the plaster down with the soap and water.
Rinse the area with clean water.
Mix 1 cup bleach into 1 gallon of water, and apply the solution with a sponge to the plaster wall surface to kill the mold.
Set up a circular fan near the wall and aim it at the wet spot. Turn the fan on to dry the surface quickly.