Valley Forge Elm lacy leaves
Hello and thank you for contacting AaE for help.It's difficult to tell from the pictures but something is definitely eating your Elm leaves. It could be Japanese Beetles but I think I see that whatever is eating the leaves is also eating the veins which Japanese beetles avoid. There are a few other insects that also chew on Elm leaves at this time of year including: Elm Sawfly, Redhumped caterpillar and Whitemarked Tussock moth.
I can't really see any culprit in the second picture but your description suggests an Elm Leaf beetle. I've attached a picture of it for you to check if that is what you saw. It is doubtful that it is this beetle as well since it also eats between the veins of the leaves.
I'm including a link to the U of MN Extension web page describing all the different leaf eating insects of Elms that have been researched that you can look over to see if any matches what you're seeing. Some of the insects identified on this page will require further research as to what you can do if one should match. Please feel free to reply with more information if you should identify your culprit among the ones on the linked page.
If it is the Japanese beetle, here is a link that talks about what we know about them and what can be done.
I hope this helps.