The photo depicts how mold will grow on a baseboard when there is high indoor humidity, or a moisture leak into a room from a wall (or both conditions.)
This mold was found in a closet. The baseboard was damp from rainwater that penetrated a crack in a stucco wall. The air inside the closet was still and the closet was dark when the door was closed. Food source for mold (wood); constant dampness, darkness and still air are the perfect breeding ground for mold colonies.
The baseboard was on an outside facing wall which was not only damp, the wall was cooler than the other walls and that too was favorable for mold amplification (growth). Cooler surfaces are more readily able to condense water from the air than warm surfaces.
If you are inspecting a property for potential mold, always look in a closet.