One possibility is bacterial infection. Fish may be susceptible to bacterial infection in inappropriate water quality, pressure, wounds or sources of infection. This infection usually leads to swelling, ulceration, discoloration and other symptoms of fish.
Another possibility is fungal infection. Fungal infection usually leads to white or turbid spots or lumps at the infected site. Fish with weak immunity are more susceptible to fungi.
You can observe whether there are any abnormalities in the behavior of clown fish, such as loss of appetite and inflexible swimming. If the clown fish is confirmed to be sick, it is recommended that you isolate it in time and consult a veterinarian or professional aquarium shop staff, who can give more specific suggestions and treatment methods.
If the above situation is not applicable, there may be other reasons leading to the abnormality of dorsal fin. But you can't make further observation and diagnosis through text information. I suggest you seek the help of professionals to solve the problem.