Hi,
I would say it is Papilloma a viral infection which tends to effect Koi in periods of lower water temperature. It is a relativly harmless disease and it does not pose a major threat to your Koi.
The average zise of the "lump" or tumour-like growths is around 5mm and is red or pink.
There is no cure for this, and it should go away as the water temperature increase.
The growth could be removed sugically.