Why is there a flaky white precipitate that forms in a gl of water when an ice cube melts?

I don't know whether this happens with all water, but it has happened with both tap water and the bottled water that I've noticed. The white precipitate does not occur in still water from the same source, but shows up after it has been frozen and then thawed. I can guess what it is, but why does it only show up after being frozen?