Hello ... Whilst it does look unsightly, what you have is some of the paint finish in the cavity wearing away - Eventually it will all wear away, but it is a cosmetic finish & there is no worries about the metal underneath rusting (unless it gets chipped like a club face of course) - Mizuno have used this type of paint finish for many years (I had some MP60 irons that were the same) - If you want to make them look "tidier" you can remove the rest of the paint finish that has worn? - Others have done that, but it is time consuming to do it without using something very abrasive or chemicals & imho not worth the effort (maybe I'm just lazy?)
HTH ... Paul
HTH ... Paul