weed in lawn
This looks like knawel. It is an annual that grows from seed. Following these lawn care tips promotes a thick lawn that reduces weed seed germination-
Please compare the photos in the links here to confirm it.
Scroll down and click each picture to enlarge it, here-
Remember to read the label and follow all precautions when using any chemicals. Also, note the caution for dicamba around trees and shrubs in the second link. Thank you.