White creamy substance (not calcium) observed when boiling water is filtered through coffee paper filter
I assure you it is calcium carbonate. 

The particles are so small they do not just sink to the bottom like a stone. Instead, they aggregate at the nearest phase boundary. All small impurities do that, unless they have a very low interfacial tension with water. It's energetically much more favourable than sinking.

Over time, this creamy substance aggregates further, up to a point where the aggregates are large and dense enough to dissociate from the surface, and sink to the bottom.