Scum is defined as a layer of unpleasant or unwanted material that has formed on the top of a liquid. The same word is derogatory when it often used to describe very bad or immoral persons. In the image above,  a rather unpleasant looking  scum has formed on the surface of a bucket of collected rainwater. The microscope reveals a different reality here in that the scum is actually a molecular forest where chains of photosynthetic cyanobacteria form green and vine-like strands in between which a thriving population of  animalcules move and hunt for prey.  This usually invisible community  is  as diverse as the visible life in any life-size forest. Please see the videos below taken at magnification of 200-times and more.