What should I do about mold in the corner of a bedroom?
Fix the underlying problem before. There is a reason why you still have mold. I would personally rip that wallpaper and locate the source of humidity/water causing this mold. If the water infiltration was fixed correctly, there shouldn't be any mold unless the affected area wasn't let to dry before sealing the area. In order to properly clean mold, you'll need to properly protect yourself N95 mask and gloves and wash thoroughly the affected area with a solution of water and bleach. 1cup of bleach per gallon of water i'd say. Good Luck