Anyone know what this brown on my sand and rocks are?

[Heathers.why]

It's new to form and coating alot of my tank. My water quality is perfect and my tank does not receive direct sunlight. I have my lights on 10 hours a day.,

 

[Crystal A]

Diatoms. Normal thing to happen to a fairly new tank. Give it time and once all the silicates have been consumed by them it will go away.