Probably diatoms if the tank is that new, im not sure if you were expecting bright white gravel to stay that way under water, it would be wishful thinking. Its like expecting your car not to get dirty if you go for a drive down the motorway in the rain. Just clean the gravel a couple of times a month with a vac.
If you have only had the tank 3 weeks, then i might ask if you have cycled the filter before adding fish? Chances are you have ammonia in the water if the diatoms are taking hold rapidly.
Got liquid test kit to test your water?
Once the filter is established with the bacteria needed to keep your tank ticking over, the diatoms should slowly disappear.