This doesn't look like MS, Mycoplasma Synoviea, although a blood test could confirm this. My flock has MS. And I can tell you that during the onset of MS and throughout the most infectious period it was running thru the flock, even the tiniest bit of swelling in the foot joints or at the webbing had my birds limping fairly badly. They limped until the swelling was completely gone. And NEVER did a single scale raise even with the most severe swelling. One thing the scales did do was go opaque. That was the first symptom I was able to detect as I followed it thru the flock, before any swelling or limping even began. I never saw any lesions or bumps on the webbing like this birds webbing has.
I have never had leg scale mites in my chickens before, HOWEVER, I have had it in my quail and yes, they have had it in on leg only. Leg scale mites move quite slowly. I never allowed my birds leg scale to get as crusty as this birds leg is, but this is what it does look like. I am surprised as bad as this condition is, that it is not in the other leg as well.
I also have some very old quail with arthritis, and it tends to effect the toes and hocks. The scales are not raised with this ailment either, although there is a lot of swelling. The limping in birds with arthritis is pretty bad. This doesn't look like arthritis.
I am no expert, but I am going to say that this is is the work of leg scale mites. I would dip this leg in olive oil, pat dry and liberally but gently rub some vaseline on the scales. Don't pick them off. The vaseline will soften them and they will fall off as the scales heal. This will take some time. I dipped the feet and applied vaseline for about 3 weeks to get my quails scales to lay back down and lose the crusts. You will probably have to do this for a few months. Apply the vaseline at least every other day for a few weeks, every day if you can. The more the vaseline stays on the scales, the faster this crud will fall off. I would also treat the other leg as I would imagine if one leg has mites, the other is well on it's way.
If you don't treat this leg scale, it will eventually cause permanent lameness and internal leg infections.
Good luck and keep us posted!