Ann Magnolia frequent powdery mildew
This looks like powdery mildew, a fungal disease on the foliage of the magnolia.
This season we have been noticing this on susceptible tree species.
We do not usually see this on magnolias. There is nothing to do now. Make sure the tree has plenty of sun and good air circulation. Rake any fallen leaves.
Monitor the health of the tree next growing season. It is possible you may not see this next year. http://extension.umd.edu/hgic/powdery-mildew-trees-and-shrubs