First, thank you for sending such clear pictures. They were key to making an identification.

The small spots on the underside of the leaves are mulberry whitefly (Tetraleurodes mori) pupae. I've never seen such a heavy infestation. Despite its common name, this whitefly can be found on a wide range of trees and shrubs. It's also a prolific producer of honeydew that gets colonized by black sooty molds.

Although your tree is suffering an unusual number of whiteflies, it doesn't mean the infestation will threaten the overall health of the tree. In fact, mulberry whitefly is mostly considered a nuisance pest.

Here's a fact sheet that talks more about this insect: https://content.ces.ncsu.edu/mulberry-whitefly#:~:text=Adult%20mulberry%20whiteflies%20are%20pale,which%20sooty%20molds%20often%20grow)