This looks very much like Japanese Stiltgrass (Microstgium vimineum). It is an annual grass that germinates in the spring and spreads by seeds that are prolific in the fall. It is shallow rooted and easy to pull where you find it in landscape beds but this is not usually practical in widespread infestations in lawns. The best control method is to apply a pre-emergence herbicide to your lawn in mid-march annually. Do this for at least two years. Pre-emergence herbicides suppress seed germination. Post-emergent herbicides for crabgrass can also be effective in killing stiltgrass.