was it recently set up? If it was, then do not worry so much about it. Every new tank goes through the process of brown algae bloom in the beginning. However, if you continue on with good water changes they should gradually disappear. Just make sure you do not add any livestock into the tank since it seems like your aquarium is not mature yet. If the brown algae problem gets serious and does not dissappear, shorten the length of time of lightings and make sure you use r/o water for water changes.