I purchased a ficus bonsai several months ago, and recently every time I water it, a white powdery mildew (I assume) grows on all over the soil surface and on the exposed roots. I unfortunately don't have access to any specialized bonsai products, and I am very cautious of using anything that could harm my bonsai. I'd greatly appreciate if anyone could tell me a simple, household solution for getting rid of the mildew.