How old is the tank? Usually algae growth is associated with excessive organic matter, which can accumulate as the tank ages. If things were going well for a long time and then started taking a turn for the worse, I suggest that you make some critical observations of the tank to identify where and how can dirt have accumulated, be it the substrate, the filter, trapped with the hardscape...
If there are no notable events that may have caused this algae issue, improved maintenance is usually the way to go. More water changes, better cleaning of the tank and filter...