Remove mould. Consider sanding down mould and removing it with either bleach or anti-mould foam.
Tooltip Tags: mould
Anti-condensation paint. Consider painting anti-condensation (e.g. DryZone by Safeguard on Amazon 5L £53.70) or anti-mould paint onto walls vulnerable to condensation or mould (top coat only).
Stop mould forming
Mould is inhibited by nitrates. Nitrates are used in foods to prolong the shelf lives of meats and prepared foods. The presence of mould in a wall exlcudes, or elminates risng damp as the root cause, as groundwater, the source of wateer in rising damp, is rich in nitrates as it passes through the soil.