Pilewort, Burnweed, remove now to keep from seeding. Erechtites hieracifolia. Here is a link that might be useful:

"The web page cannot be found" is all I get.

If it's the same stuff as what lines the roadsides in this area, it's Glandularia. Used to be included with Verbena, I think. If so, not native, but definitely ubiquitous! http://allthingsplants.com/plants/view/124328/South-American-Mock-Vervain-Glandularia-pulchella/

Becky: I think I see what the confusion is. The OP said he "just" had seed put down, and I had it in my head that this seed was still germinating, and in any case certainly is not long enough to mow. I reread his post and see that he said "in the last month" these weeds have been growing, which suggests the new grass is older than I thought. My bad. So given the grass isn't quite so new, I'd just pull the weeds out. I could get the ones in the photo in 10 minutes. I do hope the OP updates us on what happens. A little more on type of seed, etc. would be nice, too.