It is likely either diatoms or organics. The two are much the same. Both tend to remain on surfaces, but come off easily with your fingertip. To suggest treatments, we will need to know more about your tank. The photo will help, but in the meantime, if you could provide data on water parameters (pH especially, but worth knowing the GH too) and nitrate, live plants, any plant additives, fish load and feeding. Water changes (schedule and volume), and filter cleanings. Any cloudy/hazy water?
If this is either organics or diatoms, it is usually due to organics entering the tank or not being removed.
Byron.