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I had something similar looking, it was easy to scrub off with a toothbrush but came right back. I had fairly low NO3 also PO4 was hard to keep down. I think it was because this stuff was consuming a latent source of fertilizer. In my case that source was 4" MarinePure blocks. They worked great at trapping detritus in my sump and converting it to nutrients. I did a deep clean, ditched the blocks and added a rollermat. Green monster is gone now. PO4 creeping up might be leeching from rocks. They can be a giant storage buffer and re-release quickly. I did a few treatments with Phosphate Rx, and GFO reactor. I could drive PO4 down and it would rebound, do it again and it would rebound less, again, again. Once I got it to stay low (depleted the rocks), it has stayed low with moderate GFO usage for maintenance (I feed heavy).
You know, I have a large Marine Pure block in my sump. I wonder if that could be the culprit.
Api algaefix this stuff is magic