There are two types of monkeys in Mussoorie, langurs and rhesus. Langurs are beautiful, graceful, and fairly shy of people. Rhesus are very aggressive and can be dangerous. So langurs are hard to get pictures of, while rhesus may attack if photographed.
These are sometimes called Hanuman langurs, because the monkey god Hanuman is often depicted as having silver or white fur and a black face (although he’s also often portrayed as hairless or human flesh-colored).