Powdery mildew, while generally not a major concern for most succulents, can occasionally affect certain species, such as Euphorbia francoisii, particularly during the winter months. Though it's not as formidable a pest as the mealie bug, powdery mildew can still be a nuisance for succulent enthusiasts. It looks like fluffy white mold on the leaves.
Prevention can be attempted by keeping the plants relatively dry throughout winter. However, I've noticed that powdery mildew can still occur even in plants that are kept dry. When I detect powdery mildew on leaves, I typically use one of three spray treatments: 70% alcohol, a commonly available anti-fungal sulfur spray from hardware stores, or neem oil spray. All three seem to be effective, though the sulfur spray might have a slight edge in terms of efficacy. I typically only will spray the affected plants and it's neighbors.