+1 to “pitting corrosion” and its cause.the thin plating is just worn in this area. there isn't much to do about it really. as you use your mouthpiece more of its plating will wear out with time. an attempt to making this area more shiny would only aggravate the local wear.
I don't know the history of this mouthpiece but my guess would be that someone had the habit of leaving the reed installed on it after playing and the moisture in this area slowly corroded the plating.