Quoting from https://en.wikipedia.org/wiki/Yixing_clay_teapot:
Use with tea
Yixing teapots are smaller than their western counterparts as the tea is often brewed for only a few seconds before it is served to guests. Reusing the same tea leaves multiple times, the first brew of the tea leaf is usually used only to clean tea, teapot, and cups and is not to be consumed. Chinese people traditionally drink from cups that hold less than 30 ml of liquid and are simply repeatedly filled so that they may cool rapidly but can be ingested before the tea becomes cold.
I find it insufficient and misleading in more ways than it led on. While other aspects are all debatable and mostly personal preferences. One thing most tea drinkers find it fascinating but so wrong for one's health is the concept of a yixing teapot captures the flavor and color as a coating, so far as it can make tea without using any tea leaves. It became a game to build such coasting, while it is acceptable to coat the outside, but the inside? Well, the BIG question is, will you drink it????
It's nothing more than poor hygiene. We first need to know what that layer is. Clay teapot are porous, higher temperature fired, less porous, lower temperature fired, more porous. Clay is a combination of many varieties of minerals! The pore is where it traps tea and react CHEMICALLY with the elements in tea, later oxidize in dry condition, which means there are tiny residues film the wall and tugged inside the pores in the teapot. Year after year, it adds up, we call them tea stains, so by adding water alone, it will stain the water. The visual effect might become an indication of a time investing hobby, but again, is it something you want to put in your body???
Things happen, like a film of tea stains over the years, you are the one to determin is it good for your health!