I've always been curious about the build up of white "plaque" inside my hot water kettle used for coffee and tea that I clean weekly with hot vinegar. It's non harmful calcium and magnesium mineral deposits. I notice it build up in my silicone water bottle too. I'm usually using filtered spring water so this makes sense. https://www.kitchenaid.com/pinch-of-help/countertop-appliances/how-to-clean-an-electric-kettle.html