Is it "brown" algae or blue-algae (that can look like brown-algae too).
How old is you tank? Your water param.? If the tank is new, brown algea is common (brown algae is like pioneer "plant"). Or if your tank is old and water values are good, then brown algae is caused e.g. by too little light.
You didn't give any specific information of your tank: size, light, water values, fishes, water changes etc...