This looks like a type of soft scale insect. Magnolia can be susceptible to magnolia scale or tuliptree scale.  The scale sucks sap from twigs and small branches from May through early August. They exude a honeydew as they feed which fosters the growth of a black sooty mold. The bees are attracted to the honeydew. 

Treatment is the same for either scale. Sprays are ineffective when eggs are present from August through September. At this point, we recommend that you handpick the scale if possible and drop into a bucket of soapy water. 

You can spray a dormant rate of horticultural oil in the dormant season after the leaves have dropped from deciduous trees. This should help to reduce the population. Follow label directions. https://extension.umd.edu/resource/soft-scales-trees-and-shrubs

Keep the tree watered during dry periods and make sure mulch is no thicker than several inches and away from the base of the stems.

Marian