White stuff on magnolia bark
You have Magnolia Scale. Within these white armor bumps are small insects that are chewing at the soft moist layers of the tree. Here are a few options for you.
A small infestation, (such as on one limb only), the limb may be cut off and that will prevent further infestation. Be sure to completely destroy the limb,
Horticulture oils for summer use, may be applied as directed on the label. They sort of smother the insects and may take a while to penetrate the armor.
And if you wish to use an insecticide I suggest that you look for a "Yard and Garden Insect Killer" that contains the active ingredient imidacloprid. For it be effective you would have to mix in a bucket and pour around the tree as a soil drench. The insecticide is taken up into the vascular system of the plant and poisons the scale. This can be done in August and September. Please follow the instructions on the label.
I hope this helps.