Bit fuzzy, but it could be corn speedwell, the winter annual, which begins to produce seed for the following fall verysoon after vertical growth first evident in spring. IF so, combo of garden variety post emergent lawn weed killer suitable for your zoysia ASAP, and a preemergent this fall (or even late this summer, depending on where you are) may be best bet ( I think). Get other opinions please. I thought vigorous zoysia wouldn’t allow it to become established.