Any chance you can get a picture that is clear? Very hard to tell what we are looking at.
Whatever it is....no, I don't think you should just euthanize the koi. If it's something viral, it will likely come and go with temperature changes and may or may not return. I have stuff pop up now and then and once it is there, it's there. If it's hikui, you don't need to euthanize.
Can you please provide water parameters for:
ammonia
nitrite
pH
KH
Off topic comment....the koi looks very thin from the sideways photo I can see.
Andrea
Koi Health Care Committee Member