White stains that make potatoes sticking to my iron frying pan
These are hard-water deposits, mineral residue from your water that can build up over time, and interfere with the pan's non-stick properties.

You can get rid of them with a paste of equal parts baking soda and vinegar applied to the discoloration, and left to do it's work a half hour or so. Attack it with a nylon scouring pad, and finish off with a thorough rinse and dry. 

You shouldn't need to re-season the pan when you're finished, but a fresh coat of oil and some time in the oven afterwards couldn't hurt.