Mine is not a self cleaning iron and it does the same thing yours does (hard water I think causes it) and as you said, fortunately it washes out....
For kicks I clicked on one of the links below the thread and found this....very interesting and it says to clean an iron use a burst of steam and it helps somewhat.....I will try that next time I use mine...here's the link in case its disappeared from below.
http://www.consumerreports.org/cro/appliances