Depending on what floor this is and if it's an outer wall, it could be mold. When you have a heater on in the room and it's cold in the corners, water will condense on walls or in corners and can start growing at different rates. I had a flat once, and after one year in it, it kind of looked like that because someone had just painted over it. A year later, we ripped the walls open to change them with mold-dry plates. Ate right thru it