Cold surface meets humid air, basically.
A possible cause if it's has an outside wall involved, is that an area like the eaves doesn't have insulation and will be colder.
I have seen it in cupboards where ventilation is not too good.
In one really bad case, a neighbour asked me look at water coming through the bedroom ceiling, thinking it was roof defect.
When I looked in the roof space, it looked like a tropical rain forest and everything from roof felt to insulation was dripping wet with condensation.
The family felt the cold and had additional propane heaters and also did a lot of cooking.
They had had extra insulation put in without checking or improving the roof space ventilation.
In another similar incident, the cold water tank was insufficiently insulated in the now colder roof space thanks to the insulation on the bedroom ceilings. That also came though the bedroom ceiling.
What to do?
You either need to improve ventilation or investigate if there is a cold spot up in the roof space above the mouldy area.
It would be better to improve the cold surface/humidity problem at source than try to cover it with anti-mould paint etc.
One avenue would be to look at thin insulation products that you can apply like thick wallpaper.
Having had a closer look at your photo, it doesn't look the classic black mould that is spottier in appearance.
It might be worth testing the area with an electronic damp detector to see if it's just surface stuff of is of deeper origin.
