Plasterboard Mould

Bought a house, main house is lovely but it has a outhouse/mancave built by the previous owner (a builder himself apparently) looked like a strip and redo job but taking everything back to the plasterboard revealed this mold coming from the bathroom and spreading through the plasterboard from there outwards. So it's only in one corner of the building, would I need to strip all the plaster board out?

I also see that he's just dot and dabbed to the walls with no moisture barrier or insulation. I'm thinking it might be worth just getting a pro in as it's looking out of my depth, but as it's an outhouse I don't mind messing it up so much.

Any advice appreciated!