Help with tank discloration....?

I have a 55 gallon all-gl aquarium and the white rocks and white plants have some brown discoloration showing on them. It looks a lot like rust. It is also behind the heating tube on the gl. The tank inhabitants are 3 goldfish and a very large algea eater. I use a water condtioner along with a Tetra Whisper Powerfilter. Any ideas of what it is and how to get rid of it?