The quiet surface of a cup of tea always seems to be covered by a thin, immovable film, even when convection currents are still moving things around inside the liquid at some speed. The surface reflects light a bit like oil. When touching the film with a spoon it seems to crack like ice, and adheres to the spoon in flakes as a brown substance. What is this substance? Is it the main component of what stains tea cups? Does it come from the tea, or is it from the water, made visible by the colour of the tea?
The scum on the top of the tea is due to hard water (ie calcium carbonate) deposits combining with the tea and reacting with oxygen, this article has some more details if you are looking for them. I live in a hard water area and I use brita filters to get rid of some of the hardness, I know when the filter needs changing when the hard water scum comes back.
You don't have to worry about it. It's not some kind of chemical foam or impurities. It's just a kind of reaction when the leaves are infused with hot water. They call it tea scum.