Hello,
Yes, I agree that it sounds as if you have magnolia scale. Treating scale effectively relies on proper timing of the product that you use. Your best option at this point is to wait until sometime in August to spray with a product that contains bifenthrin, carbaryl, cyflulthrin, horticultural oil, insecticidal soap, malathion, or permethrin. You will begin to see the immature insects begin to emerge from under the armor you see now. Initially, they cover themselves with a white waxy protective coating, but later in August (it may be a little earlier this year), this coating begins to break down and the insects are more vulnerable to the spray. You will need to spray again a few weeks later. If it is a very heavy infestation, and you are doubtful you eradicated the scale, you may also want to do some dormant oil early next spring before the buds open or wait until May and use a root drench product containing imidacloprid. Here is a good article that explains it very well. Always read the label to find a product that is appropriate for magnolia scale and follow the directions.
https://hort.extension.wisc.edu/articles/magnolia-scale/
If you prefer to have an arborist some to your site, we always recommend consulting an ISA-certified arborist.  Find one here: 
https://www.treesaregood.org/findanarborist

I hope this is helpful.
Thank you for contacting Ask Extension.
Ruth