I'm a Chinese so I might be able to help a bit here
Wei Ling Tang is actually not one herb but a set of herbs mixed in specific propotions. The major functionality is to calm upsetting stomach and stop diarrhea. It can also be used to cure some skin deseases (don't know the English translation though) caused by "cold wetness" (a Chinese medicine term).
But it's a human medicine, and I'm not sure if it can be given to cats? Does it fit Mr Man's symptom? If you want to give it to your cat, please be careful with the portion. Also most Chinese medicine taste bitter, so you may want to figure out a way to hide the taste.