The picture of the swelling is too blurry to accurately identify the problem.  Please send a closeup of the swelling.  Also open one of the swellings and check what is inside.  Send a picture of that as well.

It is probably hackberry psyllid. Read the following website and see if it describes what you are seeing.  If it is that, then it is specific to Hackberry trees and would not affect a different tree such as elm.

https://hortnews.extension.iastate.edu/hackberry-psyllid

Thank you for the excellent pictures and description.  This appears to be Lecanium scale.  They are known to attack hackberry trees. The following websites give additional information.

1. https://extension.umn.edu/yard-and-garden-insects/scale-insects

2. https://entomology.ca.uky.edu/ef430

3. https://www.ag.ndsu.edu/cpr/forestry/begin-scouting-for-scale-insects-on-trees-07-18-19

The articles give a good explanation of controls.  You want to treat the crawlers.  Since you found powdery material, they may have already hatched. Although it is early, our hot weather may have speeded up their release. 

The third website shows you how to look for crawlers in the leaves.  A lighted hand magnifier would be helpful.  Good luck!

You are very welcome.