weed identification and its eradication
This looks like Japanese Stiltgrass which is an invasive plant giving a lot of people headaches. It is an annual weed, so if you can keep it from going to seed, do.
It will die with the frost. You will need to thicken up the grass there, and/or use a pre-emergent herbicide, similar to crabgrass control (but no fertilizer included) in early spring to prevent it next year.
Here is our page on it for more information: http://extension.umd.edu/hgic/problems/japanese-stiltgrass