First of all run bio pellets never have any algea problems (Its been working for me awhile now). Phosphate removing media is only successful when used with proper water that means no tap water, good bulbs, and a tank owner that helps clean it manualy as well as a clean up crew. Tap water can NEVER be used. Over feedind is also a good contributer to algae. Dont overthink it. I dont believe in the overnight success program never worked for me in life or aquariums!
do you baster your rocks? if so how often? also sand, how often do you disturb the sand bed? Skimmer? I assume they are the floating bags of chemipure/purigen where water just trickles over them? how big a water change are you doing as well? Consistency is important.
Sounds like you are doing everything correct. Do you make your own salt? If you wanted to do an experiment to rule out water, buy only memfish water for a while several weeks and water changes and see if it clears up? I would suggest incorporating a 2LF reactor or the like if possible, would take some headaches away. Other than that take a baseball bat 2 the thing!