Thanks for the question.

I am having trouble seeing the “mites” in your picture. However your statement that something is dropping from your elm inclines me to think that aphids are present rather than mites. Aphids can produce the leaf curling you are observing by removing water from the leaf. Additionally aphids cause the production of copious amounts of a sweet liquid called honeydew. Most likely, what you are hearing/seeing dropping from the elm is this honeydew. There is also a black mold that can be present in the leaves feeding on this honeydew. I believe this is the black substance present on the leaf in the picture you sent.

There are two common types of aphids which are found on elm trees: Woolly Elm Aphid (Eriosoma Americanum) and Woolly Apple Aphid (Eriosoma Ianigerum). This latter aphid overwinters on elm trees and in the spring travels to apple trees. I suspect that this is the aphid with which you are dealing. Both aphids have a whiteish, woolly appearance – hence their names. You should be able to see them on the leaves. Neither aphid causes significant damage to the elm. Chemical treatment is generally not needed.

https://agriculture.canada.ca/en/agricultural-production/crop-protection/diseases-and-pests-agroforestry/woolly-elm-aphid

https://bygl.osu.edu/node/1953

https://treefruit.wsu.edu/crop-protection/opm/woolly-apple-aphid/

https://hnr.k-state.edu/extension/horticulture-resource-center/common-pest-problems/documents/Elm Woolly Elm Aphid.pdf

https://www.gov.mb.ca/agriculture/crops/insects/wooly-elm-aphid.html

https://wiki.bugwood.org/Eriosoma_americanum

Please look these sites over. We would be interested in your thoughts. Most specifically, look for white, fluffy aphids on the ground, floating through the air, and/or on the leaves.

Good luck. Thanks for using our forum.