For a patch smaller than a square foot, you can handle it yourself. Wear an N95 mask and gloves. Mix one part bleach with four parts water, scrub the area thoroughly, and let it dry completely. Ensure the room is well-ventilated during the process. However, since it’s on an exterior wall and recurring, simply cleaning may not be a permanent fix. The underlying cause is likely thermal bridging and moisture seepage.

Given its location and that it’s getting worse, contacting your building’s maintenance is a prudent first step. They are responsible for the building envelope, including external walls. They should inspect for possible leaks or insulation failures from the outside. In many cases, the building’s handyman will address the symptom, not the cause.

If the problem persists after maintenance’s intervention, a professional inspection is warranted. They can use a moisture meter to quantify the dampness within the wall and identify the exact source. A lab test can also confirm if it’s Stachybotrys (the “black mould” of concern) or a more common, less hazardous species like Cladosporium. This data is crucial for ensuring the building’s fix is actually solving the problem.