Brush the affected area thoroughly with a wire brush. After the growth is removed, clean the surface with a bleaching powder solution made by mixing 50g bleaching powder in 1 litre of water. Apply on the affected area with a sponge, and leave the solution on overnight. After it dries, wipe it off with clean water. This will effectively remove the growth. However, if there’s dampness in the area, the growth may recur. Therefore it is a good idea to waterproof that entire area. Keep doors and windows open so that the area is exposed to air and sunlight.