Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Biofilm is actually a specific growth on the solid surfaces in the tank. It is a structure created by the microorganisms, including the nitrifying bacteria. It has channels built into it that allow a good flow of water to all the organisms. The term I have read for this is "extracellular polymeric substance".
At first the bacteria stick to the walls of the tank, rocks, plant leaves, ceramic merpeople and each other because they are a bit sticky. Then they start to produce this matrix that modifies their habitat just a bit.
Surface scum could be just about anything that floats on the water, and includes oils and proteins, so it is usually a bit sticky. It reduces the amount of gas exchange between water and air.