Surface scum: the neuston

Surface scum has its own community of bacteria, algae, fungi and protozoa. It even has its own ecological niche label, the "neuston." We don't encourage surface scum in the aquarium, to put it mildly. I regularly drag a sheet of paper towel across water surfaces, lifting the front edge as I go, to take up the scum, neuston or not!
 
In fairly still ("lentic") waters, surface tension creates a stable environment for organisms and offers the best access to carbon dioxide and oxygen in the air. The density of living cells on the surface may be ten to a hundred times what it is in the water layers just beneath the surface slick.
 
Many bacterial strains in this biologically-enriched surface layer carry photosynthetic pigments, though you might not notice cyanobacteria, for example, until they stain your paper towel blue-green. There are also true algae, diatoms, fungi and single-celled protozoans in these organically-enriched microlayers.
 
In still waters, the upper face of the neuston is home to many mites and some springtails. Occasionally you may see an Otocinclus flip on its back and make some passes through the nourishing oily bacterial and algal slick of the water surface. If you doubt the nutritional rewards, consider what a dangerous feeding strategy this is for the Oto in the wild.
 
Indoor pollution makes a less positive contribution to surface scum in the aquarium. Many household cleaners and air "fresheners" that mask odors impart a pine or lemon scent, which diffuses in aerosol droplets that contribute to the surface slick. Terpenes in those aerosols can even react with household ozone to lay down a thin coating of particulates. (Forest haze is formed in a comparable way, when natural terpenes react with ozone.) Oil droplets and smoke particles from cooking and heating make their own contribution, even in a house where no one smokes tobacco.