I have seen something similar but right off cannot recall what it is. Have you asked the people at your local office of your Cornell Cooperative Extension Service for help?

Here is a link that might be useful: Cornell CES

It's called Japanese Stiltgrass. As you've probably noticed, it grows very fast. The only thing I've found that will kill it in the lawn without harming the lawn is Bayer Advanced Bermudagrass Control for Lawns. It's proven to be very effective for me, and I had Stiltgrass growing everywhere in my yard.

To prevent the stuff from coming back next summer, be sure to use pre-emergent in the spring.

Here is a link that might be useful: link to Bayer's product page

Since this is an annual and will shortly be dying off because of the cold weather there may be no reason now to spray something as potent as that Bayer product now.

Here is a link that might be useful: Japanes Stiltgrass

I removed a lot of creeping charlie from my lawn last year, only to find the weed above take its place. I have planned to power rake out the stilt grass, and seed this fall, hoping to block some that way.

by "use a pre-emergent in th spring" are you referring to the Bayer or like? or does something like CGM have any effect on it? I would prefer to not use chems due to pets.