Hello dent ford and Welcome to Mrs. Clean's Forum:
First thing you or the landlord might want to check for is a leak that is causing the dampness which will cause mold and mildew to grow. If you don’t find the source, the mold and mildew will continue to come back.
I can really sympathize with your situation. I actually had a similar thing happen in one of the first houses I rented. When I moved a dresser it revealed a horrible large area of mold.
My wall was painted (not wallpapered) and since you do not specify, I'm going on the assumption that it is a painted wall. Let me know if this is not the case. I am happy to tell you that I got rid of the stain fairly easily. Below are a few things to try.
Bleach- Highly alkaline and caustic it could burn your skin so wear gloves while cleaning. Bleach instantly removes mold and its stain with minimal scrubbing. Bleach must be rinsed thoroughly off the surface being cleaned to avoid damage.
Use 1 cup bleach to 4 cups water to make the cleaning solution. Clean the wall working from the bottom up and work in sections, drying after each section is cleaned.
Borax- Borax is alkaline like bleach but is not as caustic. It removes stains and brightens without bleach. It won't take mold stains off on contact, but they should come off easier than with vinegar. Alkaline borax inhibits mold re-growth.
Borax is one of my favorite cleaning products. It's cheap and highly effective. Use 1/4 cup borax dissolved in 4 cups of HOT water to make the cleaning solution. Borax is a great mold remover and works on most surfaces.
After you have washed the area don’t put the furniture back until the wall is completely dry. If you can, move it and keep it in another area of the room. Also check out the back of the wardrobe, it may need cleaning too.
Preventing Mold and Mildew-*Keep areas that are poorly ventilated like bathrooms, closets, basement, etc., as clean as possible and disinfect regularly.
*Any kind of dirt or soil on a surface offers a buffet of food for growing mold and mildew.
*This stuff thrives in damp and dark areas of a home. Using humidifiers and dehumidifiers in your home is a great way to keep moisture levels lower.
*You still may find the nasty fungi growing in some places in your home. Whenever you find mildew stains, be sure to attack them with quickly with the methods above.
For further information
http://www.housecleaningcentral.com/en/cleaning-tips/living-area/clean-walls.html