Magnolia is difficult to manage and unfortunately, they have not found a treatment that really works.

You have to treat when the smaller scales are moving. Any treatment you choose will not affect the adults. 

If your infestation is small, know them off.you can also blast them with a strong stream of water if you catch them in the crawler stage. 

 If your infestation is getting worse, I would recommend calling a professional; especially if you have a large tree. 

Heavily infested branches can be trimmed out. Be sure to put them into bags so that they do not fly away. Left untreated, they will kill your tree. 

https://apps.extension.umn.edu/garden/diagnose/insect/trees-shrubs/not-obvious/magnoliascale.html

https://extension.umn.edu/yard-and-garden-insects/scale-insects

https://northerngardener.org/magnolia-scale/