What's this white mould?

Good evening @Markymark the powdery mildew that attacks roses is known as Sphaerotheca pannosa Powdery mildew on roses will form what looks like a white powder over the surface area of the rose leaves, and it may also spread to the stems and new rose buds. It can disfigure the leaves and they will not come back to their normal shape even after the powdery mildew has been killed. Powdery mildew on rose bushes loves to attack the fresh new foliage of the plants and will also stunt the bud growth, causing disfigured blooms and, if left unchecked, will prevent the buds from opening. Warm, dry days followed by cool, humid nights are perfect conditions for an outbreak of powdery mildew.

You can buy a fungicidal spray and use once a week until all signs of mildew are gone, use the spray evenly to foliage in the morning to avoid leaf burn. Make your own spray by combining 2 teaspoons of baking soda and 1/2 teaspoon of liquid soap in 2 quarts of water.