I would assume it is actually dried up water. And the white powder is the "hard water" residue. You probably have "hard water" in your area.
I notice under my fridge a "White Powder" I moved the fridge and didn't see a leak. Of course, I never notice it before. What could it be?
If you live in an apartment they may have come to treat for bugs. Your landlord or property manager should have notified you though.
It's those darn ants. They raid the fridge at night and then snort coke under it. You are seeing the residuals of their parties.
