There is no root cause. Algae grows everywhere where light, nutrients, space, trace elements, and lack of herbivores are present.
Unless you plan to starve your tank and kill your corals, there is no greater cause.
To be frank, I get frustrated at posts attempting to claim nutrients are the cause of algae issues. You can find many great tanks with no algae with higher nutrients. You can also find tanks infested with algae despite overusing GFO and carbon dosing to deplete nutrients.
You can grow algae at 0.03ppm equally well than at 0.20ppm. The PO4 is not limiting. More won’t increase growth because something else will be limiting.
I'm not suggesting that the elimination of nutrients will solve the problem.
Based on the information presented here in the thread, it seems to me the root cause is elevated nutrients, not the presence of.