Looks like spurge. Use broadleaf weedkiller. You will get best results when temps are 90 and above. Apply pre emergent 2-3 times a year. Where are you located?
This stuff is hard to get rid of. I have heard it spreads through spores, so mowing over it spreads it around. I had a bermuda 419 lawn that constantly had this in it and it seems like it never goes away. If the yard is small enough, hand pull very carefully so as not to disturb things. It has a single root, so it is easy to get a big chunk with one pull
was watching videos from "lawn care nut" and he has some great videos on lawn care. Used to work for True Green. He recommended Bayer Lawn Weed and Crabgrass killer concentrate, with a surfactant such as baby shampoo because the leaves have a wax coating which prevents the herbicide from being absorbed.