I find that many Rusty-Brown spots are growing on my Rocks and even on the front glass. What are these ? Some kind of Algae ? How do I fight them back and how to remove them ?
It's algae... you can reduce the amount of food and light in the tank, and that will help. Scrape off the stuff on the glass with a sponge or a plastic card...library card works pretty well.
I had that problem too a little while ago (new tank) then it turned to green algae, I reduced the amount of light during the day now I have that in check and everything is going well in my tank.
If the rock has enough iron as part of its composition, it will exhibit brown, rust like (oxidation). If it is upsetting the chemistry of your water, you can try limestone or some other type of rock/mineral.