What is in the grass?
The weed looks like it may be a type of pennywort or dollarweed, Hydrocotyle spp.
It is a warm-season perennial weed that spreads by seeds, rhizomes, and tubers. Dollarweed thrives in weak, thin turf in moist areas where the drainage is poor. It is a difficult weed to manage.
The first step is proper lawn management practices,
Scroll down to page 3 of the following PDF for additional information on controlling it with a herbicide. It can be spot treated with a liquid herbicide. This will help to manage it but total control may not be possible.
2, 4-D and dicamba are found in many liquid lawn herbicides labeled for home lawns. Read and follow all label directions.