Hello Rachel, happy to help.

Your magnolia probably has magnolia scale.  It's an insect that, in its mature stage, appears to be a pinkish, gray shell-like formation on the twigs and branches of the tree.  They excrete a sweet sticky substance that attracts sugar loving insects and also lands on the lower leaves, branches, and plants below the tree.  The substance then starts to mildew and turns into what we term as sooty mold.  It's all unsightly but harmless to the other plants.  The scale insect that causes this can multiple exponentially very quickly and overwhelm a tree.  Here is a link about magnolia scale with some information about its control which is tricky because you have to time it just right to get the crawlers that hatch in late summer or early fall.  The specific information about magnolia scale can be found under the section entitled, Soft Scales on Deciduous Trees, then scroll down some more.

Please note that any use of a pesticide requires the responsibility of reading all instructions on the label to make sure it's the right chemical and to learn when, how, and how much to apply it safely.  More is never better.  

Good-luck