Every summer I take my poinsettias outside. Within 2-3 weeks of bringing them indoors for winter a white powder appears on the leaves. I am able to get them to bloom but the leaves eventually yellow and drop off.
Powdery mildew is the culprit. This fungal disease is common where light and air circulation is limited. Try moving your poinsettias to a sunnier window or under artificial lights. Make sure there is plenty of space between plants. This is often difficult when we have to cram our plants together in front of any available light source. This is usually enough to correct the problem.