mouldy2shoes

I noted you stored the plasterboard outside albeit with a waterproof cover. When it rains, the relative humidity can easily hit 95% outside, even if it was lower than this, the cooling of the pile of plasterboard each evening would mean the RH within the paper on the plasterboard would be sufficient to start mould growth without it being readily visible. The moment the board was applied to the adhesive effectively provided sufficient moisture for the mould to prosper. In any case whilst you may have enjoyed mould free plasterboard for some considerable time, applying plasterboard to brick walls is a bad idea as no matter how dry a brick wall feels, bricks always hold moisture and one has to consider that behind the plasterboard one has provided a pocket of warm stale air. If you turned a refrigerator off even if empty and clean and left the door closed upon return months later you shall be met with a musty odour, so why should it be any different in the situation of dot and dab. Once you have visible black mould (likely the toxic starchybotrys chartarum as this loves paper) on your plasterboard it requires ripping out. Traditionally for several hundred years up until the 1950s houses were completely lime plastered, the lime is too alkaline whilst wet for any mould to prosper, once dried it still isn't a very hospitable substrate for mould to thrive upon, I would suggest after ripping out the plasterboard, you either render the wall with lime cement or indeed lime plaster. Lime plaster will absorb moisture on humid days and release it on drier days, although many modern paints will prevent thus from happening and they themselves can allow mould to prosper underneath the paint surface. Consider either wallpaper or a limewash finish to that lime plastered wall.