White powder on the foliage of plants is most often a sign of mildew, a fungus that thrives where there is poor air circulation, weak plants and too much moisture in the air or in the soil from poor drainage. Look closely before you say for sure that it is a fungal jungle out there. Sometimes insects, such as scale, mites or even aphids, look a lot like white powder. Either way, cut off the worst of the infected leaves and then wash any remaining foliage with a mild Ivory soap solution and rinse with water. You can make your own indoor fungicide spray by adding one tablespoon of baking soda to one gallon of water and then add one teaspoon of vegetable oil to make the mix stick to the plant leaves, unless the plant is flowering... then id just cut off the infected area and salvage what you can
A university study agrees that this baking soda and water mix has proven just as effective as any fungicide spray you can buy for houseplants. But you are not out of the jungle yet. The mildew will roar right back if you don't change the environment by making sure the plants don't sit in water, letting the soil dry out a bit between watering and use a fan or crack a door or window on mild days to increase air circulation.
Like I said however, make sure mold is what youre dealing with first before you apply the foliar spray.