The most common causes of mold growing on walls are high humidity, condensation and water leaks (which are often hidden inside the wall).

Condensation forms when water vapor in the air meets cold surfaces and cools to become liquid. Condensation is especially common on walls which form the perimeter of a house. These walls are often colder because of cool outdoor air on the other side.
Things like drying wet clothes inside or steam from hot water increases the humidity in the home. If humidity stays high in your home for a long time then you might see mold starting to grow on your walls. Usually the best way to decrease the humidity is to ventilate your home by opening windows and doors.
Leaking pipes near or inside of walls are a common cause of mold. If there’s a leaking pipe inside a wall cavity then mold will usually be well and truly set in before it’s discovered.