welcome to a new tank.. 100% normal..
diatoms are almost a brown dust and can easily be blown off with a turkey baster..
They will go away with time as they are feeding on excess silicate in the tank..
as stated could also be a hair algae,etc..
Part of a new tank is diatoms/green/brown algae,etc.. and usually they all go away with time provided you are maintaining proper water quality..