Annual bluegrass seed head. Most people control it by applying pre-emergent ( weed preventer ) in the fall ( early fall or late summer depending on your location) and winter..... It dies off in the heat, and returns in cooler weather. The Trimec won't touch it ! To reduce seeds falling into the desirable turf, use a catcher when you mow.

And mow your grass more often

Tenacity will kill it but it takes multiple apps and the stuff will be dropping seeds all along. A good pre-m in early September and again in late winter will help providing you don't need to overseed in the the fall.