i have the same problem, its brown type of dust. its on the leaves i try to clean it by rubbing each leave but its alot of them it comes off but not all the way, is there a better way to get rid off that ?
Snails, shrimp, and otocinclus will eat it. Increasing the amount of light will often help but usually leads to green dust algae instead. If you are feeding fish decreasing the amount of food will often help. Increasing the amount of flow and filtration will help. Oxygenating the water, say by emptying 50% of the water and running a powerhead above the surface once a week will often cure the problem. Time is often a cure but not always.
See i dont have a clue of what causes it, i lowered amount of food, i do a regular weekly 25% water change, and i srub every last bit of it off the glass and rocks, and filters(not very hard cuz it comes right off, actually only takes a few min), but slowly it comes back. The tank is 45g.