Dichanthelium clandestinum; Poaceae (grass family)
Remember, I am not a botanist. But I’m reasonably sure I’ve identified the species correctly, and quite sure I’ve gotten the genus right. There are 22 species of Dichanthelium found just within the boundaries of Washington, DC!
This thing was a real bear to photograph. It was almost impossible to get the actual flowers in focus. I admit this photo saw a lot of touching-up in Lightroom.