Hi Kevin and thanks for sending in your question, but I really need to see pictures of the tree and a close up of the leaves affected as well as any healthy leaves. I'll attach the OSU site on the Elm leafminer in case you have already had a professional diagnose the problem and just want a treatment. 

Pay particular attention to the Cultural Control section and then skip to the Chemical Control table 3 to find four chemicals you can use on leafhoppers: carbaryl, clothianidin S imidacloprid S  and pyrethroids.
(The S means it is a systemic that will go through the roots of the tree but often kills more than just the insect you are aiming at.)

Look for products in your garden store that have one of these chemicals and say they are recommended for leafhoppers. Be sure to read and follow the directions for use VERY carefully. This is the law.
https://pnwhandbooks.org/insect/hort/landscape/hosts-pests-landscape-plants/elm-ulmus-elm-leafminer
Be safe,