MrWhiteCat has a large grey smudge on his tail, near the base. I expect he was under a car and brushed against something greasy and grimy. He's middle-aged, and I don't think he can reach to groom that part any more (or he's just a lazybones!). I have to brush that area frequently, otherwise it gets yellowish and dandruffy.
I've brushed and brushed, and most of the mark has gone, but there's still a greyness there. Given that he's not likely to lick it, do I need to completely clean the stain off, or is it OK to leave it? And if I need to clean it off - how?