A grimy, black biofilm is starting to cover the Jefferson Memorial, and it can’t be killed

They don't know what's causing it? It's called "not regularly scheduled cleaning". They will more than likely end up using some sort of enzyme mixture to disrupt and breakdown the extracellular polymeric substances (EPS) that is protecting the bacteria. Enzymes work by bio-degrading the organic materials that the mildew/bacteria feeds on to grow. They are as well pH neutral and work great on natural stone.

That is sad news CPT Jack Durish. Hopefully the museum curators and the National Park Service will soon discover how to remove the black biofilm for good :-)