Scale insects need to be treated while they are small and mobile. This will likely occur around August. Because of the size of your tree it will require commercial spraying equipment to treat the tree. I would recommend consulting with an arborist. Local arborists can be found at treesaregood.org.

https://hort.extension.wisc.edu/articles/magnolia-scale/

Magnolia scale does not typically result in the death of a tree, but can contribute to long-term decline, eventually killing the tree or making it susceptible to other diseases and injury.