I can't see the spot on his leg very well, so I won't comment on that. The browning of green C. ceratophrys is natural, but it may be a wound getting infected on your frog, I can't tell. I agree with DVirginiana on the points he made.

I was looking at your care information. There are a few things I wanted to point out. Firstly, I don't believe that the substrate you are using is safe. Bark is dangerous because it doesn't break down if swallowed, and it has plenty of jagged edges. Pacman Frogs are a burrowing species. It's possible that he may have gotten that spot on his leg from trying to burrow (they back up into their burrows). I recommend changing out all the bark and replacing it with a product like cocofiber (found at Petsmart, PETCO, etc.). That will retain humidity, and give him a safe material to burrow.

Secondly, I noticed that you aren't giving him a calcium supplement. Pacman Frogs *need* a calcium supplement. Your pacman frog is most likely fairly calcium deficient. I recommend going to your nearest pet store and grabbing a Calcium with D3 (they need D3 to metabolize the calcium, as they don't get it from the sun/a lamp like some reptiles). It would also help if you got a Multi-Vitamin supplement, as those are important for Pacman Frog's health.

Lastly, I noticed that you hadn't changed the substrate to date (or at least hadn't recorded it). I am not sure how that works with bark, as I have never kept bark. If you are going to switch to cocofiber (which I highly recommend), you'll need to do a complete change every 2 months at maximum. Otherwise, it starts to sour, and you may end up with critters that you don't want in your pacman frog tank. I also recommend changing out the water anytime that you see your Pacman entered the dish.

Maybe not a lot to help with your current problem, but hopefully this will help with general maintenance. Hope this helps!