This is a condition called powdery mildew. Purchase the all-natural fungicide called copper. Follow the label directions and spray three times, seven days apart. Also, be on the lookout for small iridescent beetles that could be appearing from now until approximately July. They seem to die as soon as I plant the seedlings.
Full sun is best.
If there happens to be a little late afternoon shade, this will be fine also. If your soil is extra sandy, you may want to add some peat moss to help hold moisture. And don't forget, seedlings can be sprayed with copper too. Just follow label directions exactly. I'm having a hard time with philodendrons.