In your area of world you could have annual and/or perennial Crab Grass. The single most bestest way to eliminate it is to pull the grass out as it germinates. Spraying any poison will only kill the plant you see and do nothing for those that will germinate, so once the growing plants have been removed heavily mulch over that area to kep more seeds from germinating.

giants, we know that you can't mulch heavily over your St. Augustine! But you can apply pre-emergent herbicides at the appropriate time of year. Call your extension office for some calendar dates; locations will vary according to climate. You might even ask someone in the extension office if there have been any promising break-thoughs on organic crab grass control.