What exactly has happened to the leg? Is it missing or just bent back? If bent back, it sounds like a physical injury that probably wouldn't result from the other animals in the tank. Do you have a dwarf African frog (ADF) or an African clawed frog (ACF). ADF's are much smaller. They may be harmed by larger ghost shrimp. Mine live with some red-fronted shrimp without a problem. ACF's grow larger and do eat fish and probably shrimp too.
If the frog's leg is bent back and broken, I doubt there's anything you can do. Only a few fish/aquatic vets exist in the US that might also treat a tiny frog like that. He may be able to function like that and otherwise live a somewhat normal life. My page on ADF's and ACF's is at
http://www.fishpondinfo.com/frog4.htmAll you can do is maintain a clean and healthy tank with good nutrition and proper tankmates. If the frog's leg becomes infected with bacteria or fungus, you can try the appropriate fish medications for treatment but hopefully he'll be okay. Good luck!