Shotweed, Cardamine hirsuta, aka hairy bittercress (or a vey closely related species). A cool season annual that can be easily pulled in moist soils. Any broadleaved lawn weed herbicide will work but try to pull them or kill them before they flower and set seed (which yours have already done) as it has a large seed bank and as the common name implies propels ripe seeds over a large distance.

Photo is not great but it appears to be chickweed, Stellaria media.

Where do you live? Some weeds are easier to rule out if you live where it's cold or hot.