how young is the tank?? may either be diatoms or cyano... some pics would help give a positive id but if the tank is still pretty new than diatoms are normal at this stage if its cyano than than get your phos in check/ increase the flow to the areas where it is
watch the amount you are feeding, increase the flow as stated. If it is cyano it is usually red and slimy.....diatoms are rusty and can be blown off with a turkey baster?