[tang daddy]

Well if your tank is new like 3 weeks old then that's normal, the sand bed is new so it's gonna have to cycle, your just gonna have to ride it out. Usually when people start with new substrate they usually experience hair algae for a cycle then later it disappears and if your lucky you skip diatoms or cyano as you sand populates with bacteria.

Make sure you try to turn off flow and powerheads when you feed so that it doesn't go flying everywhere in the tank fueling the algae.