The stain is from what?

The contractors use tiles cleaner to clean. It is actually diluted HCl (hydrochloride acid) which can easily be bought in hardware store. I think mostly because to clean cement used in grouting and also used to remove stain from welding (dissolved iron stain).

Did you look at the fridge placement requirement like how much space need to be at rear, right and left. Modern fridge has heater to sidewall and also in water tank at the back to evaporate water. Did you switch off the fridge for long leave? Maybe water overflow from container at the back? Probably just water stain but hard to remove?

Wooooo ... leaking piping in kitchen. Find from where it is originate. If from behind fridge, then need to move it out to trace 

Bathroom/Toilet behind the fridge wall?