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Cyanobacteria has always been rust-colored like that in my tank and grown primarily on the glass and sand rather than the rockwork. When I get diatom blooms after dosing especially high silica concentrations, it shows up on my rockwork first and tends to be slightly more yellow/gold than in that photo. Totally anecdotal and just my opinion... but given the age of your tank I'd guess that's one of the many species of cyano. You could always pick up a silica test kit if in doubt, though. The reason I'm skeptical that it's diatoms is that most established tanks will see almost all of their free silica used almost as quickly as it's added because sponges in the rockwork can pull it out of the water extremely fast.
If it is cyano, Chemiclean works wonders and is usually reef-safe if not dosed in excess, in a well-oxygenated tank, and followed by a large water change. Some people have reported fish losses, though that seems to be rare.
Now that I think about it, I have had quite a few red spots popping up on my rocks. I used to think it was red coralline algae but now I am wondering. However it doesn’t feel slimy or come off the rocks easily? I scraped it with my finger nails and it didn’t scrape off.
I will try to reduce nutrients, do large water changes and blow off the rocks frequently and see if that changes things. Want to try the natural path first and then use Chemiclean as a last resort.



