Quick question for the Rumble Knifesmiths

So I made this Bowie inspired knife many years ago. It was the first time I made one of these so please forgive it’s crudeness.

I’ve notice that there is a constant discolouration appearing on the metal and I would like to see if I can stop it.

I suspect it’s from the sheath I made for it so I will need to make another one at some point.

Any recommendations on how to clean the blade up? I was going to try polishing it with a light polish like Brasso or Silvo and see how it turns out.