Repaint or scum? What is going on here?

This is a stock wheel from a 2003 Toyota 4-Runner. You can see where I've started cleaning it where it shines and other areas not so much. I run my fingernail and the "scum" does not budge. Is this some sort of build-up I can get off with a product or is the alloy wheel damaged and needs to be re-painted/coated? What do you recommend? Thanks for any help you can offer.

 

What I think is happening is that silver powder coating without clear over time and with use and cleaning the powder tends to dull from abrasion. I think that is the darker areas. They would need to be refinished to be perfect. I hope this helps.

 

Yes, the finish on the wheels appears to have failed/is failing. Those wheels will need to be re-finished in order to get them looking good again. Sorry to say...

 

I was afraid of that...glad I asked before I spent any more labor LOL. Appreciate the help.