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.

Nutsedge is what ya got . 'Ortho nutsedge killer' at Home depot , and it may need repeat sprays each year. If they don't have that particular Ortho product at your home depot , you can order it on-line and have it delivered to your store for free. Get the one with the hose end sprayer . Your probably going to have to re-seed again in the fall........depending on where you live, make sure you seed at least 6 weeks before the first average frost date.

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.