iNaturalist suggested blunt spikerush (Eleocharis obtusa), but this seems shorter, and the flowers seem to match slender clubrush (Isolepis cernua) more.
Could still potentially be blunt spikerush. According to https://www.illinoiswildflowers.info/grasses/plants/bl_spikerush.htm
"This plant is a summer annual about 2-10" tall."