I make this soup from a very simple recipe - except from a little butter, it's vegan:
1 yellow onion 1 garlic clove 2 medium potatoes 1 bunch of parsley with the stems removed 1 bouillon cube 1 1.2 liter of water
I sweat the onions / butter / garlic - I then add the potatoes and the water - bring it to a boil, let it cook for 15 minutes then add the parsley and the bouillon cube. I let it cook for another 5 minutes - then I blend everything in the vitamix. (blender)
The soup tastes delicious and is creamy for having no cream, however, this thin film develops on top of the soup. What is this? Is the starch from the potatoes separating and sticking together? Is there any way to get rid of it?
Since you mention the film develops after cooling, it is likely to be a very thin layer of fat.
There are some sources of fat in your recipe: the butter you sweat the onions and garlic in, and the bouillon cube. Both of these are very minor sources of fat, but it's still present. A quick stir after warming should incorporate it back in, or you can instead skim it off when the soup is completely cooled.