What is causing this

[Budman's Corals]

Been set up for a year all parameters are perfect. O2 on phosphate. See the brown stuff on the sand by the rocks is it diatoms and why am I getting them

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[cymaster007]

Looks like either cyano or diatoms. That area may have low water flow across it, when residual fish food settles there it starts to decompose and feeds the bacteria and other organisms in the area causing a small bloom.