The black algae probably is eating your chlorine, but just for reference, could you please post a set of test results? FC, TC or CC, pH, TA, CYA, and what you've used for chlorination?

For now, the best thing you can do is get it up to shock level and keep it there, not allowing the Cl to yo=yo up and down. I'll let others chime in about how well the black algae treatments work. Black algae is very stubborn and resilient, so it's not going to be an easy fight!

Janet