Your tank appears to be rather heavily stocked if not overstocked

I wouldn't suggest adding more fish at this time.
Algae usually grows due to excess nutrients(fish food, fish poo etc) or excess light(bright light from a window or sunlight) or inadequate water changes and substrate vacuums. Maybe a combination of all three. We've all been there at one time or another

Since a fishes stomach is the size of one eye it's really easy to over feed. On a tank that stocked at least 25-50% water changes and substrate vacuums need to be done weekly to keep the Nitrates(waste, dead plant leaves) down. This is algaes favorite food.
There are several things that can be done but first Here's a link
http://www.plantgeek.net/article_viewer.php?id=9 look and see if you can find your algae and lets go from there.
Carol