While cleaning the headlights, I find this impurity on the inner surface, but not sure what that is. If I have to clean it, the only option I have is to take the glass off?

Rudy

 

Are you sure it is inside? It looks like the classic failure of UV coating and worsening cloudiness of the lens.

I have found 3M glass polisher does a great job, quickly, for one step cleaning and polishing of the lens.

If it is indeed the inside, I would bite the bullet and buy new ones while you still can get OEM.

 

I had similar stuff. I had them proffesionally sanded and clear coated. most of it is gone now.