There are different H’mong tribes in Sapa, however, the two most popular ones are the Red and Black. The one in this picture is a Black H’mong. These H’mong people, I believe, originated from China and Sapa is close to the Chinese border. You can read more about them in this Wikipedia link.
At Sapa, when you walk around the main street, there are many of these H’mong sellers, who try to sell you something, and if you just had an intention of looking at them, either being interested in what they sell, or just wanting to take pictures of them, they would follow you everywhere until you bought something. So if you did not want to be bothered, I would recommend that you don’t show any interest in them at all. For me, since I wanted to take their pictures, I had to buy something from them even if I was not interested in what they sell.
We also had an opportunity to visit one of the red Hmong village, unfortunately, it had been raining before we got there, so the main village road was muddy and wet. We took only a few pictures and left, nevertheless it was a good visit and experience.