There are two separate problems here and only one of them is yours. The one that is yours is that you are self-conscious about the mark on your leg. I get that, I have a big birthmark on my leg, and it can make me self-conscious too. So yes, look into concealers and so on. Sounds like you already have that under review.
The other problem is that your co-worker is obsessed with the mark on your leg. Guess what? Not your problem. She can find whatever method works for her to get over it. It only becomes your problem when she goes on and even on about it, and then your problem is that she is being rude and boring, not the issue over which she is being rude and boring.
I would call her on it, coldly. 'You seem to have an inordinate, and frankly rather disturbing, interest in my legs. You may be quite assured that I have taken all the medical advice that I need or want. I understand that you have some problem with this, but frankly it is just that - your problem - and I would appreciate you dealing with it without mentioning it to me again.' Then any time she raises it, ask, still coldly, 'are you obsessing about my legs again?'
Backup method: draw up a five by five grid on a piece of paper, and put it, prominently, on your office wall. At the bottom of it, put a picture of some treat: bottle of wine, expensive meal, new book, sexy shoes, whatever floats your boat. Every time Miss Does-Not-Engage-Brain-Before-Speaking comments on your leg, put a large X in one of the squares. Complete the sheet and you get to buy yourself the book/shoes/bottle of wine. Extra points if somebody else at work asks what the countdown is for, because office gossip being what it is, if you say, even without mentioning names, that you're counting the number of times somebody makes an impertinent comment about your physical appearance, and that you've nearly won a magnum of champagne, it will get around.
Backup method 2: boredom. 'Oooh, DottyG, your poor leg, it looks so...' and you interrupt. Aim for the tone in which a teenager comments on the sheer uncoolth of your music preferences. 'Yes, I know, I've got wet rot, dry rot and death watch beetle, my leg is going to fall off any day now.' This is one circumstance in which rolling your eyes is perfectly acceptable. Actually, I think this one might work best. She's plainly not troubled about being rude, but nobody likes to be thought boring.