Lokii,
You need to determine what type of algae you are battling. GFO may take out phosphates, but there may be certain macroalgae that require minimal phosphates, but love nitrates.
Same reason why people with biopellets still see issues with phosphates, because the bacterial uptake of nitrate to phosphate is in a certain ratio, and the food we put into our tank may have a disproportionate amount of phosphate to nitrates.
Plus, I have read that too little phosphate just causes all your stuff to "not grow".
But more hermit crabs and/or add a tang to help with the algae problems