White Coolant Residue

A few days ago I was doing a regular check of fluids and left the radiator cap off. After driving a couple of miles I smelled it, pulled
over and replaced the cap. Back home after it cooled I went to check the level and saw this residue...
Photo color may not be true -- actual color is a little darker than milk cream.
Any idea what that could be? Radiator was flushed out and pressure checked about 3 months ago and had the same thing. Radiator shop said he had no idea what it was.
Tastes like a metallic salt to me.
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