It’s not clear what’s covering the Jefferson Memorial, why it’s there, or if it can be killed, but the National Park Service is trying to figure out how to get rid of it.
According to The Washington Post, biofilm, which is "part algae, part bacteria, part fungi," has begun to cover the surface of the memorial, especially on the dome, around the base, and on the triangular pediment. Currently, the National Park Service is experimenting with what cleaning solutions will remove the biofilm without harming the marble structure. It’s not yet known if the biofilm damages stone.
Other structures in the D.C. area with biofilm covering them include the memorial amphitheater at Arlington National Cemetery, the Washington Monument, and the Lincoln Memorial, reported The Washington Post.