I think it hold the moss together and makes it easier to wrap it well in plastic. Algae does grow on anything damp in our climate. I was taught years ago that roots prefer to grow in the dark - like that provided by a flower pot or propagation flat hence keeping the plastic covered. All the air layers I saw at the American Camellias Society gardens were covered with aluminum foil. There were shiny little "packages" on many old plants. I have used black plastic, covering the clear, for the same purpose. I like to keep the clear plastic intact so I can see through it and there is no chance I disturb any roots. Like you, I have been around awhile. My parents taught me to air layer by putting a brick on a lower branch and leaving it touching the ground until roots formed. That was more than 60 years ago. Every climate is different and what works for me might not be what works for others in different locales.
Good growing on your fiddle leaf!
