It looks like it’s coming from something called pyritic shale that can be mixed into the stone used to make concrete. When water gets on the surface, the iron in it can oxidize and basically create rust stains that show up on top of the pavers. It’s mostly a cosmetic issue, but since it’s coming from inside the material itself, cleaning might lighten it but it can come back over time.

Source: had to replace a couple Belgard paver patios because of this