I have some scale similarly in a couple small areas where water doesn’t flow, but regularly wets. My guess is the water hits the areas, evaporates, and leaves the calcium behind. Day after day, the little bit of calcium builds and becomes visible. Likely unavoidable. I have not addressed it yet (will leave it be until my wife asks about it lol), but plan for those areas is a scrub with dilute acid.
For waterline, I’ve seen some youtube “miracle” videos where folks raise the water level enough to cover the scale line and then use Orenda SC-1000 per directions. Over time, the videos show the scale softening enough to where it can be brushed off. I’d be interested to know if the experts here consider it snake oil or a legitimate option. I’ve been thinking about trying it myself next Spring. They bill the SC-1000 as a chelator and calcium being a metal, it sort of makes sense but that’s as far as I know.