What is this?

[SaraB]

So I decided to turn my air stone off when I added my new oto cats to the 55 gallon tank until they were settled in.
Today I decided to turn it back on and came home to this. What is it? Has anyone else had anything like it? All my fish are still alive and active (thankfully)

I did previously have some Indian almond leaves in there but they never stained the sand before.

 

[Lokilow]

Looks like rust but its likely just some algae since there wasnt much water movement.

 

[SaraB]

I agree that it looks like rust but I don't have anything metal in my tank. I didn't see it yesterday when I cleaned the tank either that's why I didn't think it could be algae.