trying to identify this weed
The lower half of this photo is my zoysia grass, and the upper half is what looks to me like a different kind of grass but have been told is actually a weed. The weed is slightly darker and has much thinner blades, but it seems to grow and spread just like grass. It is taking hold in this area that has poor drainage and where the zoysia is thin and not healthy. Can anyone tell me what this weed is, so I can find the right herbicide to take care of it? Many thanks!