Shot weed, Cardamine hirsuta. An annual that grows very early season and has an ability to 'shoot' its seed pods - those long skinny things at the tip of the plant - considerable distances, hence the common name :- ) And it produces seeds in copious abundance!! Ideally, you want to remove as soon as you see the foliage rosette forming in early spring - very shallow rooted and pulls up easily - but certainly once you see any flowers forming and before it goes to seed.

Now all you can do is treat with any lawn broadleaved weed killer. But as it has already dispersed a huge number of seeds that will not be addressed by this, prepare to reapply several more times as they continue to germinate before the weather heats up. And it will likely reappear in fall as well.

Never seen that before. Thanks Gardengal for the specifics.