We occasionally see algae that is VERY resistant to chlorine, but not often enough to really know a lot about it, except that higher chlorine levels will kill it, if you sustain them for awhile.

I've dealt with that personally, 2x, on very large pools, and my priority was always kill it NOW, and get it cleaned up. In one case, I poured (2) 55 gallon drums of 15% bleach into a 160,000 gallon pool . . . and the green stuff died RIGHT THE HECK NOW. But, that was a 100+ ppm dose of chlorine in a pool with CYA < 30 ppm. (Don't do that in a vinyl pool!!) Obviously, that's not an optimized dose; 50 ppm might well have been more than enough. But I had a community pool to open, not a test pool to experiment with.

Anyhow, that's the sort of input that leaves us lacking in real specific advice, except the advice I gave myself: "Never let this happen again!"