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?
Does this tweeter use ferro-fluid (used as a suspension agent)?
Perhaps its containment has ruptured allowing the fluid to seep unto the driver?
Maybe it could be a heat related discoloration; I think roxy54 suggestion is a good one or calling a Avalon Dealer and getting their opinion as well.
Avalon's distributor can't do anything about this (especially I bought this in 2nd hand market). I just sent email to Avalon, I have found from internet a lot of complain about this problem in my area (Hong Kong).
Pull it, test it,find model num, if bad find equivalent