Any chance of pictures?

If you are asking about lichens those are harmless. If you are asking about bacterial leaf spot that is an infestation. But to get rid of it you will have to find out for sure if that is what you have, then undertake a specifically targeted spray program. Targeted as in using the right chemical in the right concentration at the right times.

Otherwise magnolias like better soil conditions than many other plants will put up with, if yours seems stunted or has poor yellowish leaf color try top dressing with composted manure. If it is a small specimen you can probably find enough of the product in bags at local retailers. A bigger job and it starts to become more economical to buy it in bulk. All the more so if you have a truck, can have the bulk product loaded into that instead of paying a bark and soil dealer to bring it out to your place in a bigger truck.