Water flow is probably a major cause here, but I'll tell you what I actually found to work for me.
I had an algae problem that was 10x worse than yours. At first, I listened to everyone else and tried everything on the difficult road to fixing my algae problem. Many water changes, more flow, decreased lighting, RO water (of course this was part of it), etc. Than I finally broke down and tried the easy road. I bought Algaefix from API and it worked it's magic. You add the fluid solution to the tank every 3 days until the algae is reduced and then just add once a week to maintain. In two weeks, the algae was gone. I had numerous, massive hair algae all over the place. I tried buying numerous snails, scarlett hermit crabs, lawnmower blennie (although I love the guy), and many others to try to eliminate the algae. And all it took was to buy the Algaefix when everyone mentioned that it wouldn't work.
Now, you should definitely get more water flow in your tank and use RO water whenever possible. And yes, the Drinking water from Walmart is RO filtered. It mentions that on the label. But I would definitely try Algaefix from API and see if it fixes your problem along with more water flow.
Hope this helps and good luck!
