Is the tank newly set up? If you rub the leaf between your fingers does the brown just rub off. If so you have diatoms. Diatoms are self limiting and once the excess silicates in the water are used up they will die out. I've found running lighting only 6 hours during the diatom phase keep growth down. Also you can add some nerite snails, which can't reproduce in fresh water, and they will help keep diatoms cleaned up.