Make a small hole in the drywall with a drywall saw. Use a small flashlight to peer into the hole to see if there are any plumbing or electrical components behind the wall.
Put on protective gloves, safety goggles and a respirator.
Cut away all of the affected drywall with the saw. Place the pieces in a plastic trash bag and seal it. Once mold has attached itself to drywall, the drywall must be discarded and replaced.
Check for leaks. Fix any leaky conditions and mop up any standing water.
Check the wooden studs behind the wall for mold. If mold is present, clean the area with soapy water or a solution of one part bleach to 10 parts water. Allow the area to dry thoroughly.
Replace the affected drywall.