Don't cut the light back any further...your coral(s) will suffer too much. The plant nutrients are coming into your tank in this order of probability:
Feeding too much, most probable cause. Cut back on feeding rather than light.
Using city water for makeup water, it can be loaded with nutrients, use RO
and good quality salt mix.
Not enough filtration, a protein skimmer will help reduce nutrient loads and excess fish food.
Algae eaters only reprocess the nutrients and algae continues to grow. Good for some fish, but you know the downsides.
Keep up the water changes, especially after cleaning the tank.
There is no healthy reef in the world that is algae free. It is part of the whole process, but you want to strive to strike a balance with it.