What you refer to is probably scale, an insect.  I will give you a link but be aware that the only way to deal with it is to treat your tree with a product that is taken up throughout the entire tree.  Your bees will also be affected is they partake of your blooms.  With such a large infestation, it may be better to take your tree down this fall.  Open this link and then the one on Magnolia Scale.  https://extension.umn.edu/yard-and-garden-insects/scale-insects