Mold growing behind loose wallpaper can prove a difficult beast to tackle. Even if you remove the obvious, visible mould, spores may have worked their way into other areas of the wall. However, if the wallpaper only became damp in one section, chances are good that the mould is contained to that portion of the wall. While you can clean and disinfect mould behind wallpaper with bleach and water before it spreads, there is no way to do so without removing the wallpaper over the mould.
Dry the wall if it is damp. Leave the heat turned on in the room, or place a space heater a couple of feet away from the wall. Dry the wallpaper slowly to prevent the plaster underneath from cracking.
Put on a protective face mask and rubber or plastic gloves. Cut away the wallpaper over the mould carefully using a razor blade. Seal the removed wallpaper in a plastic bag to avoid spreading mould spores.
Stir 3/4-cup bleach into 1 gallon of water. Dip a sponge into the solution, and then scrub the mould vigorously.
Dampen a clean cloth with cold water. Wring the cloth out to remove most of the water, leaving the cloth moist, but not wet. Wipe the bleach and mould off the wall with the cloth.
Leave the heat on. Allow the wall to dry completely again.
Replace the wallpaper over the patch of cleaned wall. Use a mildew-resistant paste to apply the wallpaper.