Hello
Thanks for your question.
Your magnolia has magnolia scale which is what you're describing as the white 'mold'. It's actually the female stage of this scale insect. As the scale feeds on the leaves and tissues of the magnolia plant, it creates the presence of a sweet, sticky substance called honeydew. This is what the wasps are buzzing around and are attracted to. The black substance that arrives soon after is called sooty mold.
There are a few different treatment and control options for you. It looks/sounds like you have more than just a few branches infected, so pruning out and destroying the infected branches is probably not an option. Other choices include using a systemic soil drench in late August, a summer soil in August when crawlers emerge, and oils applied in fall and spring. Be sure you read and follow the label as this is the law. Read more about specific treatments in this MSU Extension article:  https://www.canr.msu.edu/news/check_magnolias_for_magnolia_scale_now