It's like a type of film of seaweed, i tried stirring up the sand to break it apart and i think it makes it worse. When the lights are off - it's not as bad, then as the light goes on, it slowly darkens at the surface of the sand. I don't feed frozen food, only dry flakes and pellets. I did lower my pumps to get more flow at the bottom of the tank, i hope it helps it out.
Is there any CUC member i can use to clean this, I have turbos but doesn't seem like to do the job.