I've seen springs in lower hoses dissolve like that before--I think what happens is the spring makes contact with the aluminum radiator and touches the water pump outlet which is probably steel,and it creates a battery of sorts,and electrolosis takes over..or maybe the coolant acts as a conductor...
I have lots of iron in my tap (well) water--so much so I hate to use it with anti-freeze in vehicles--it turns everything rusty brown and likes to clog up things......what you think is calcium may be corrosion from the aluminum in your case..