Good call Johnny Hank, it is Scleranthus annuus, German Knotgrass or "Knawel." This small and easily overlooked plant grows all over North America in dry sand and gravelly places. It is not native here. The flowers are green and "apetalous" (without petals), and it's a member of the Caryophyllaceae (Pink Family). Just as the common name says, it is "not grass."
Can you name this plant? Feel free to name it or just take a guess. Good luck!