Hello, and thanks for using the Ask an Expert System,
I believe that the weed in your photographs is Japanese Stiltgrass. This is a noxious invasive weed that can be very difficult to control. Since it is a grass, broadleaf weed killers typically used on grass lawns will not touch it. You can find more information on this plant and control measures in the Penn State Extension publication Japanese stiltgrass (Microstegium vimineum).
If you have additional questions you can contact a Rutgers’ Master Gardener hotline.