Had some show up this past summer when I let my tank lapse a bit and my N03 and P04 jumped up. I got my bio pellet reactor revved up again and GFO later, and that stopped it. I still had to scrub rocks with a tooth brush just before each water change, but it loosened its hold and came off more easily after the nutrients came down again.
It does take some patience though, and even today, there are still a few stubborn tuffs I find here and there, and I have to apply the toothbrush. The good news is it can be beaten back once the nutrients are back down (N03 and P04 at zero).
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