Hello Chelsea

It may be black knot, a disease of plums and cherries. You may attach some pictures of the growths and we may be able to confirm that is what it is.

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Here are pictures of black knot and how to treat trees

https://www.canr.msu.edu/news/your_plum_trees_versus_black_knot

My apologies, I thought I had attached a picture with the original question. The substance I'm referring to is the white scales and fuzz that is on the branches. It is killing off the plant slowly, having returned from last year. 

Thank you for the photos. This is white peach scale, a scale insect pest of peach and other stone fruit, and of Prunus species, including your sand cherry Prunus x cistena

https://extension.umd.edu/resource/white-prunicola-scale/

On ornamental trees with no edible fruits you can use a systemic insecticide to control scale. Tree drenches and granular products with the active ingredient imidacloprid are examples. Read the label and choose one that states it controls scale insects. The label will indicate how much you need based on your tree’s size.

The above link notes at what growing degree days(GDD) the crawlers are vulnerable to spray treatments. For Michigan your timing of treatments are around 513 GDD ( about mid May) and 1637 GDD. ( about early July).

The GDD varies each year( this year we are running ‘early’) and for Michigan can be viewed at GDDTracker. This article discusses what GDD is, and how it is used for turf. But, you may use the tool to show you when the GDD for treating the scale occurs in your zip code area.

https://www.canr.msu.edu/news/use_gddtracker_to_schedule_applications_to_turfgrass

If you need help using GDDTracker, please call our Garden Hotline so you can speak with a Master Gardener over the phone. They are open Monday, Wednesday, Friday from 9 to Noon.<personal data hidden>

If you decide to use the systemic drench( or granules), you won’t need to use GDD since the insecticide will persist for the season. It takes some time for the systemic to move through the tree so, this year you could spray the crawlers and use the soil drench for ongoing control of adults and crawlers. Expect to treat your tree this year and next year. Then monitor new growth and only continue to treat the tree if more scale shows on new branches. (Old, dead scales dry up and fall off or can be gently rubbed off, being careful not to scrape the bark)

Another option is to hire a certified arborist to treat your tree. Find them by zip code search here- http://www.treesaregood.org

Please read and follow all directions and precautions on any chemical or treatment you use.

Thanks, again! 
Laura S.