If 50/50 acid didn't touch it, it's probably not calcium. Do three things:

1. Scrape off some scale, put in an a glass glass, and add a bit of acid. If it doesn't dissolve . . . it's not calcium carbonate (lime) and probably not calcium anything. If it's NOT calcium carbonate, we need to figure out what it is, and how it's getting into your pool.

2. Assuming that it's not calcium carbonate, go to your water company's website, and get their latest analysis -- post the link here.

3. Take some pictures, and sent them to poolforum AT gmail DOT com, and I'll post them.