diatoms or cyno. Hard to tell without a picture up, but let me calm your nerves. This is perfectly normal in all new tanks. Make sure you have decent flow in the tank, water change regularly, do not over feed, and in time this will pass.......only to be replaced by more algae!!! Again, that's normal too.
You'll go through several phases of algae before the tank matures past that point. Make sure you have a decent algae crew, detritus crew, and something sifting through that sand (not a sand sifting starfish though).
It's nothing to worry about yet. If you see this in a year from now, then worry.