There’s two options you can go with here, you can simply clean down the mould area affected with mould and mildew cleaner and paint over with mould paint. this of course would be the most budget friendly option, but there always runs the risk of the mould returning. instead we could remove the plasterboard, install storm dry on the block work and replace the plasterboards. this option would mean it’s less likely for the mould to return, but it’s up to the customer. both jobs would be finished professionally and will remove the mould from the affected area.