It is usually a bacterial infection of the cornea. You could try LFH Clear eyes drops in isolated cases, if a few fish have the same symptoms it could be worth trying a mild bactericide but keep an eye on you parameters as they can not back your filter bacteria.
I know what you mean. Some one tried stealing our 32" Chagoi and they must have dropped her on the edge of the coping around the pond breaking several scales clean in half. It became a nightmare having to treat her every few days, must have been kids as they stole the garden furniture at the same time.
We have since added pir sensors all over the place that set an alarm off in the house should they decide to visit again, we'll be ready for them next time!!!
That looks ok peter no sign of reddness or swelling, if it has no other symptoms I would leave alone, when the temperature rises im sure it will clear up on its own.