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It's about nutrients. Namely nitrate and/or phosphate. You can't see them on your test kit because they are bound by the algae. Until you get control over them the algae will continue to grow.
Make sure your skimmer is working well, using GAC and and GFO and changing it often, do a lot of large water changes (20 gallons won't do, more like 40-60), vacuum the gravel and rock a lot, change filter socks every day or so, remove as much algae as possible, put macroalgae in you sump (lighted fuge), dose kalk, etc.
You'll have to do this to an extreme until the algae is gone and then keep up maintenance to keep it away.
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