That is an impossible one to even come close to an answer without seeing, touching, scrapings for culture - particularly fungal cultures with microscopy for fungal hyphae or spores - and possibly (even likely) a biopsy ... etc etc etc.
Sending a picture wont help much, nor is it likely to get you closer to the diagnosis - or its' treatment.
Have you discussed this with your nephrologist, or a primary care physician? If they haven't provided a satisfactory answer, then have you seen a dermatologist? Has it been biopsied? What treatments have already been tried?
This is not a question that can be answered over the Internet. It needs vision, hands on, and possibly a tissue diagnosis. At first reading, it does not sound like an issue primarily with the fistula, but with the skin - the dermis. It might be an area of chronic inflammation overlying, for example, a superficial tributary vein venous thrombosis, but that is for a hands-on doctor to assess. There are so many options in diagnosis here, most of which are unlikely to be directly related to the fistula.
I would suggest that you get it seen to ... and stop casting wildly about on the Internet for advice. It won't be productive. A dermatologist would be a good place to start.