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.