Stencils put up on weather-exposed walls will eventually fade away and die of old age – if not buffed. The quality of the paint, the material in the wall, climate and level of exposure will determine the pace, and how it is fading. Like a dying / dead tree often has a certain beauty, so can be the case with street art. Especially the ones slowly transforming into something close to old cave paintings – where you still can imagine how they were in their original state, and through this look into the world of the artist.
In this case, the white paint clearly was less resistant, giving way for the black to stand alone. If the black is used for the details-layer it usually performs well alone (in general white seems to be the hardest colour to manufacture as spray-paint, the saying is that if the white is good you may safely buy the rest of the brand’s colour range).