Your main problem is a serious case of spider mites. You can see the fine webbing and if you looked closely or with magnification you may be able to see them sliding within the webs. You could also do a 'tap test' where you firmly shake plant stems over a white piece of paper and look for tiny dots walking.)
Spider mite populations tend to explode when conditions are hot and dry. Are you able to vent the greenhouse setup (roll up the sides?) to allow some more air circulation? 

Your second set of photos may show an infection, but it is difficult to tell that close-up and without seeing the top side of the leaf. (We don't think it's the bacterial infection called angular leafspot, downy mildew has not been confirmed yet in MD in 2024.)

These pages will guide you on these issues: 
https://extension.umd.edu/resources/yard-garden/vegetables/cucumbers/


Christine