I am going to guess that the tan dusty substance is a rust disease, a fungal infection encouraged by poor air circulation, too much water and moisture on the leaves. Improve these things and spray your houseplant with a fungicide to control this rust invasion. Do not let your plant sit in a saucer of drainage water. I must say that without a photo of the problem I am making a guess here. Can the tan powder be rubbed off easily? If so it could be pollen drift from a nearby plant or some other air borne pollutant. Rust disease will stick firmly to the foliage so give it the swipe test before you spray. Keep growing, Marianne Binetti