It's a very light dusting of diatoms. I wouldn't worry, it'll get worse before it gets better most likely. As the tank matures, it'll become less likely to see an outbreak. If you keep nutrients low so you're removing the food diatoms feed off, it'll make it hard for it to stick around.
Also some clean up crew will turn the sand bed over, keeping it white. If nothing is turning it over, it'll go dark overtime anyway as it'll get the same type of algae you have to clean off your glass on it.