If this is a freshly cycled tank the I would guess that you have overloaded your bio load perhaps to quickly, but that is just a guess at this point.
Diatoms normally happen only at the first few months of a tanks life, after that they are very seldom seen, which tips us off to it being a new tank.
So once again how old is the tank?
What is your water source?
Run some carbon and be ready for a water change, if this is a case of overstock to soon then you may see that ammonia suddenly spike.
Tell us more about the tank, filtration, lighting, age and all and we will go from there.
Thomas