It is Carp Pox, a viral infection usually occurring during the winter. It creates hard waxy lumps, normally on the fins, but sometimes also on the body and head. Papillomas are similar but normally smaller.
There is no cure, however, it will disappear in summer and might come back next winter. They normally outgrow it and it is not contagious and it does not affect the koi's health.
It can be surgically removed although I do not recommend it due to possible infections.