The pests pictured are scale insects, and given the Magnolia host, are probably Magnolia Scale or its look-alike Tuliptree Scale. (Magnolia is in the same plant family as Tulip Poplar.) Information for each can be found in the linked page, which also connects to a general scale page that includes management tips. Unfortunately, there is no practical control option for an extensive infestation outside of insecticide use, even if that is restricted to just lower-toxicity choices like dormant oil. Older plants like this Magnolia are too large and full of branches for enough scale to be removed physically by gently scrubbing them off.

The good news is that the population doesn't appear to be very heavy, and if the plant has been growing well, intervention may not be needed. (If it is attempted, be aware that it might take over a year for a moderate to high population of scale to get under control from any approach used.) Scale sometimes outbreaks on plants under stress, such as from drought or heavy pruning, though we can't determine what the cause was in this situation. (Maybe the plant was purchased with a very small population, and only recently have they become more numerous. It's not uncommon for scale, which blend-in well, to escape notice at nurseries or spread from existing trees in the landscape to newer installations.)

As another sap-sucking insect, aphids can also contribute to honeydew and sooty mold on foliage or bark, but we don't see aphids in the photos. If they are present, they probably do not need controlling, and between them, the scale are the larger contributors to honeydew and the resulting sooty mold in this case.

Miri