Rust Color On Sand ?

[crash4444]

ive had my tank set up for over a year . This is new to me , there is a fine layer of what looks like rust forming across the top layer of sand. Any suggestions would be great thanks

 

[xxBarneyxx]

Two things spring to mind.

Diatoms and Cyanobacteria.

Have a look in this over current thread to get an idea of what diatoms look like:
http/www.fishforums.net/index.php?/topic/374839-diatoms-taking-over-tank/

Diatoms aren't so common in stable mature tanks but can still happen, especially if you have made any big changes recently to the tank.

Cyanobacteria tends to grow in loss, slimy mats and is generally caused by high phosphate levels.

Can you get any photos of it? Also have you got any test kits to test your water (ammonia, PH, SG, nitrate and phosphate ideally).