Your weed appears to be dollar weed. I have had a infestation in one of my accounts with many weeds. I have used SpeedZone Southern with good results you can wipe out an infestation in two weeks completely. You can also try straight 2-4-D just follow the labels for mixing directions. You can use these products on many grasses with out to much harm. You can not find these products at your local home depot or lowes. Go to a irrigation store in your local area and they will have this herbicide on the shelves.
Hope this helps.