Yellow Dust. Its a bit of nasty stuff and can cause various reactions in people. You may have seen pics of Asians wearing masks, well this stuff is why.
If you look closely at the red bricks, just above the cement square with the hold, you will notice a slight bit of lighter(yellow) colouring along the edges of two bricks.. That would be the yellow dust as it is being washed away by the rain. And yes, that is some of the stuff you probably heard about in the news that was carrying some minute level of radiation from Japan.