My ceramic tweeter change to dark color - Help

After using my Avalon Ascendant II for 2 years, it’s easy to see the ceramic tweeter change to dark color. It look like oxidize or mold, but when I bought this speaker it look very nice. I know many Ascendant (I & II) has this problem, and not sure if this due to humid environment or not, that’s’ why I always use dehumidifier at home (I have 4 dehumidifier), but finally my speaker’s tweeter still change to dark color.

Is it oxidization? How can I remove this dark color? Will my speaker’s sound quality change if it became more & more dark?