Black mould on plasterboard

[grifter]

This section of dot and dabbed plasterboard was getting covered in mold repeatedly. I've removed it and the back of the board was black.
It's only this section. Is moisture possibly absorbing through the brickwork and what do l do to prevent it? It's a conservatory wall.
Pictures show where it is situated.
Thanks

 

[Wayners]

That down pipe disappears under decking.
What's going on there.
Is all the water going in a drain and free flowing away or is it blocked?
Any cracks in drain gully?
Sometimes cracks or overspill causes water to run under the foundations or in cavity..