White stickey substance
Likely magnolia scale, a small insect but there are often many of them. The sticky stuff is their excrement, or "honeydew." At some point black sooty mold often forms on the honeydew making everything look worse yet. Read more here: https://www.canr.msu.edu/news/check_magnolias_for_magnolia_scale_now
You can treat it yourself as above but timing is critical so you might want to involve a Certified Arborist, one who specializes in everything woody. If so, you can find one or more Certified Arborists in your zip code at
www.treesaregood.com Click on the Find an Arborist tab. Arborist evaluations are not expensive and he/she would look at everything else in your landscape while there as well.