This is scale, this is a normal phenomenon. It should be that you have been using tap water to make tea, which will accumulate scale. If you change the mineral water to boil tea in the future, this will basically not happen, and the boiled tea will taste more fragrant. I recommend two methods of scale removal: 1. Soak in vinegar for a while, and you can easily remove the scale; 2. Scrub with toothpaste and cleaning cloth, and then rinse with water.