It sounds like powdery mildew, although perhaps someone spilt some flour on it, or maybe it is something else entirely!
A photo would be very useful!
If it is powdery mildew, then you can try spraying with Potassium bicarbonate, or in my experience, just plain water is just about as effective. The oft repeated cause is that it is due to high humidity/dampness, but in my and my horticulturist wife's experience, it usually occurs on leaves that are dry. Wetting the leaves reasonably often stops it forming... but dont wet them too much, as that may cause other problems.
If the leaves are badly affected, the best thing to do it remove them entirely, as they wont recover, but try not to sprinkle the clean leaves with fungal spores.