The fish in the fish pond have white hair.

No, that's disgusting. This is a fungus that grows on me. That fungus will corrode fish scales. Fish will jump up and down and itch. As long as a week, as short as more than a month, fish will be tortured to death. You have to buy a bottle of liquid medicine, which is sold in the flower and bird market. Not expensive. It's only a few dollars. Change water for it regularly. Also, the fish tank should be thoroughly disinfected. Don't feed too much fish food, the rest is easy to breed bacteria. The fish that my family used to keep suffered from this disease. This fish is very painful. I hope your fish will recover.