How many watts of light are over your tank and what type (compact fluorescent, reg. fluorescent, incandescent....)? How long do you leave your lights on for?
Algae appears for various reasons, normally its a nutrient imbalance, but diatom algae can be caused by too little light, and other things.
If its brown diatom algae, it is common in newer tanks. Is your tank new?
How often do you feed?
I am battling brown diatoms in my new cichlid tank. Its very new though (about 3 weeks). I am making sure I do 50% weekly water changes, wipe off the glass, and feed every other day. It has slowed down the growth alot by doing this. If it's diatoms, it should go away on its own eventually.
I agree that you are stocked and I wouldn't add anymore fish to the tank. Just be sure to scrub it off the glass if it bothers you. Keep up with maintenance and it will hopefully go away.