Hi David
I believe it is a tumour as it is only on the side.
You cannot do anything about it. The koi might survive for years or it will just die on you anytime.
If it deteriorates, I would cull it and put it out of it's misery.
_________________
Paul Viljoen
E Mail: koi@absamail.co.za ; http://www.koionline.co.za
Koi are not my whole life but make my life whole