What is on my sand?

[MJPO]

So this is a moderately planted tank that was set up on June 29th. What is on my sand? It's been ever so slowly spreading but then in some places it ignore completely. Diatoms? Maybe water flow is a cause? I have pretty good filtration and a 700GPH circulation pump. Thanks for the input! [emoji2]

 

[TMaier]

Seems to pop up in newish tanks and go away on its own. I had it in mine.

 

[Coyne]

Looks like diatoms from the picture. Super common in new set ups. It will die off on its own once it's food source is exhausted. A nerite snail will make short work of it, but then you'll start seeing its infertile eggs show up on the decorations