So I have this older knife that has been a little neglected lately and needs to be cleaned up. I've used Mother's on it for this very same problem in the past but I'm just wondering if there is something better. Also, is there anything I can use to clean the inside of the sheath with? I think that's where the problem lies.

Don't store the knife in the sheath.

Verdigris The Emerald Lady.

Don't store your knife in the sheath. Acetic acid in the tanning compounds are reacting with the copper in the brass.

I just wipe out the sheath with a damp cloth when I forget and leave a knife in a sheath for long period.

Plenty of products including common home goods like alcohol will clean off the green.

I like Brasso.

OOOO steel wool makes it shine.