It is not nanobots, it is cyanide laced China dust. He got sprayed with some moist dust from a mister. The black dots are where the dust particles are and the light areas are just the liquids mainly. Dust particles hold more material and get imbedded into the skin because the dust is actually a fossilized larva. Notice how it is on one arm, a sign of a burst from a mister probably as he worked outside maybe cutting the lawn or something.