You might try putting diluted white vinegar into it and running it through on high heat to remove whatever sediment or particles are causing the staining. I used distilled or filtered water for my iron, back in the days when I ironed more. I think the water quality affects how the iron works.
It's limescale inside the iron. Do you have instructions for it? Mine you have to turn up to the hottest temp, with a small amount of water, then hold down a special button while waving it about over the sink. It makes lots of steam spit out of the holes, taking all the mank out with it.