Kind of hard to see, but this is some graffiti on a seat on one of the 14th Street buses. I see graffiti reading "CTU" around the neighborhood, as it's one of the local "crews" (aka small gangs), but now I know the meaning - Clifton Terrace University. I assume that's the Clifton Terrace buildings and University Place, a short street that runs between Fairmont and Clifton between 14th and 15th Streets. The other tags on this seat say 14-n-R.
Anyway, if you see any gang graffiti around the neighborhood, like CTU or LC, you can report it to the city and they'll clean it off - src.dc.gov.