i cant really tell. my all out guess is that the fish got hurt and you see the result. as long as the water quality is great, the fish should get better with no issues. if the water and stress is too much, the fish could get a bacterial infection on the wound. my advice is to make sure the water is perfect and you dont add any more fish. feeding a varied diet to boost the immune system is also a great idea to be proactive about the wound.