Sorry in advance for the photos. This is a very difficult thing to photograph as it's best seen in person by tilting the bracket at different angles in the light.

I've seen this before and usually just throw away the bracket if it occurs on a slot cover since it stands out. But it isn't uncommon to encounter it on card brackets.

What is it? I'm pretty sure this is not dirt or grime that can be cleaned (I think I've tried to wash it off in the past for other instances). Is it just oxidation?