Thank you for your question and picture for Extension. It appears your magnolia tree has magnolia scale, a type of insect that feeds off your tree, forming a hard shell at one stage and produces a sticky substance called honeydew. It seems to be appearing more and more lately in this area and is difficult to get rid of. Here's an article from Extension on scale:

Scale insects on Minnesota trees and shrubs | UMN Extension

For more information from Wisconsin Extension:

Magnolia Scale – Wisconsin Horticulture

Treatment with pesticides is difficult since there are many stages of the insect and timing is crucial. If you are dedicated to saving your tree, a certified arborist is your solution:

How to hire a tree care professional | UMN Extension