@Rich17 your koi has a bad case of cotton wool fungus which you should treat immediately , the fungus is a secondary problem that has been allowed in because somethinghas damaged the protective mucus all fish have either by bad netting techniques or damage through jumping poorly maintained ponds and or filters spawning or poor diet parasitic infections and bacterial infections .
I would however suggest before treating this koi you must first identify that which has allowed the fungus to become a problem otherwise your peeing against the wind in treating the fungus outbreak.
Small areas of infection can be treated in the pond but large areas such as we see on this koi should be seperated off to a Quarantine tank before treatment.
The best way to treat these infections is by one of the many fungal remidies out there on the market which have differing levels of success the best way however is to topically treat the koi by first sedating it then with a cotton bug remove the fungus with a cooton wool bud then malachite green should be applied to the area before spraying the area with propolis .
However seeing as the infection is around the gill area care should be taken when applying the malachite green lest it do damage to the fishes gills.
Remember in future that prevention by keeping a clean system water filters etc , plus obeying the QT rules is the best way not to get fungal infections in the first place
Dave