better ventilation really and a fungicidal /black mould [bathroom type spray] cleaner. We have a similar problem in the 'out place' and the moisture that the plants produce is partly the problem. I have found washing the window sills with mild bleach solution before bring the plants in helps a lot.

Ventilation is key for preventing black mold (maybe install the ceiling fan). Also check to see if you have water seeping in someplace - around the window frame or from the roof. Prolonged dampness, water saturation and standing water favor black mold and it spreads quickly. Black mold can be a bio hazard if it gets out of control. Definitely wear a face mask It can cause respiratory problems if inhaled. It should be eradicated el pronto.

You can get rid of black mold with vinegar. Here's how...

Vinegar. White distilled vinegar is an affordable, natural solution to removing black mold. Its antibacterial acidic characteristics are exactly what you need to get the job done. Pour the undiluted vinegar into a spray bottle to apply to the area, or just go for it and pour that vinegar right onto the mold stains.Apr 30, 2020.

https://www.countryliving.com/home-maintenance/cleaning/a27379221/black-mold-removal/

Thanks for that and yes there is a small concrete larder next to it and that walls tend sweat in winter.