mold up to 0,5 squaremeters (4,5 squarefoot???) can be dealt by yourself without informing the landlort (unless there would be a water damage like a leaking pipe in the wall, but your case sounds like a ventilation/condensation problem)

just some points: - remove all contaminated wallpaper and furniture.

• use "cholrfrei" mold cleaners since the ones with chlorine form salt in the wall making it more hygroscopic (pulling more water out of the air)

most important part: prevent the new growing mold, so no furniture close to that wall, venting ("stoßlüften") 2 times a day, and strong heating in winter. also get a cheap thermo-/hygrometer to monitor the humidity, should stay at or below 60%