I had a that same problem on the drivers side of my car, but worse. My car had mudflaps on the front and they just kept water in them and it rusted the bottom part of the fender, and part of the bottom rocker panel. I ended up replacing the front left panel, having the rocker panel sandblasted, and buying the m-tech sideskirts which I then bolted over the old rocker panels. You should get that rust taken care of immediately, though. You don't want it spreading anywhere else.
If you don't care about replacing the grey, sandblast that as soon as possible and repaint it grey.