It sounds like mustard algae, which is the most chlorine resistant form of algae.

You'll need to raise your chlorine level to at least 20% of your CYA level to control it . . . but you have to be able to (a) accurately test your CYA level, and then (b) accurately test chlorine levels above 10 ppm.

Using a phosphate remover can help, since very low PO4 levels slow algae growth, but ONLY if you get PO4 levels below 125 ppb. To accomplish that, you need ANOTHER testkit, to measure phosphate levels. Unless you do this, phosphate removers are a waste of time and money, since lowering PO4 levels from 5,000 ppb (common, with some city water supplies) to 1,000 ppb accomplishes nothing, since 1,000 ppb is STILL more than enough for the algae.