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[katmet2017]

Anyone know what this could be? Cyno or what?

 

[Reefkeepers Archive]

Cyanno or diatoms, completely harmless and will usually settle itself out after the tank matures. Just part of the "ugly stage", don't worry about it.

Also my advice would be to remove that fake plant

 

[alanr3]

Could also be precipitating calcium on your sand bed due to lower flow areas and the resulting low flow is probably making the Cyano thrive there. Saying Cyano because of a similar patching pattern and color (you will need to verify with microscope)