Your valve core should be brass. Calcium has little effect on them.Time for a new tube. It is good to hear of other that replace cores. I replace them every year to every other.
You are correct on brass but Not sure where the rest of the information is from but they do corrode and start seeping over time and best preventative maintenance is to replace them periodically. Otherwise it is a full tube replacement not just a core.Your valve core should be brass. Calcium has little effect on them.
That rubber o ring seal does not last indefinitely and neither does the core in the core seals. Not uncommon to remove the valve cap on it is full of water. The core of the core is the most common failure point. On the main core I treat the o ring and just a bit of anti seize on threads when I swap them out. Because I change mine frequent there might be some green on the core. When I first started changing them had some the ends were missing they were so old.Valve cores need replacing occasionally because of hardening of their elastomer seal stretched over the metal support.
If metal corrosion is an issue, then your tube’s valve stem interior is just as likely or even more likely, to be corroding, as is, the valve core material. The core is only 1/2 the seal design