Mildewcide is an additive mixed into paint that inhibits mildew and mold growth on the finish. Damp or shaded areas, like north-facing walls, benefit most from it because the additive keeps the coating itself from becoming a breeding ground.
Mildew grows on paint film, not through it, which is why shaded, moisture-prone walls go green while sunny ones don’t. Paint with mildewcide slows the growth, though no additive replaces fixing a drainage or ventilation problem that keeps a wall damp.