There are some simple steps to take if the tap water is hard (alkaline and limey). First, contact or go online and look for the specs from your provider's water testing. If you are at the very high end, consider using a filter on the tap or letting the watering can sit for a day before use. Much of the lime will settle to the bottom so don't use the last one inch in the can. Here are some other measures: http://homeguides.sfgate.com/water-plants-alkaline-water-tap-water-70007.html
Without a picture, I am only guessing but I hope it is an educated guess. I, too, have hard water but this has never led to a problem but I have in-ground roses. You may have "rose scale" which produces a whitish crusting on stems. There are many kinds of scale but take a look at white rose scale images to rule this problem out.
