With cyanuric acid (CYA) levels near zero, you lose chlorine to sunlight very quickly. Because of that it is easiest to fight algae in the evenings until you can get the CYA level up to the 30-50 range.
Your PH is a bit low at 6.8. You should bring that up to at least 7.2.
The key to fighting algae is to give it some concentrated attention, adding chlorine every hour or so, for several hours in the evening. You want to bring the FC level up to between 10 and 15 as frequently as possible, up to every half hour, for several hours one evening, and then maintain it around there each morning and evening till the water clears up. This is simplest to do with a good test kit. Without the good test kit you need to do a little guess work and use a little extra chlorine.
For more information on fighting algae see
this article at Pool School.