I've cleaning up old saddles for a little while now. My favorite thing to do is cleaning the ones that have been left to collect dust. So I've seen alot, but that is not mildew or mold. You have a stain in the leather. It may or probably wont(saying that bc it's really really dark). You can try to darken the saddle (olive oil will not darken leather-unless it is very very dry-then the leather is actually lighter than it originally was, and adding olive oil brings the color back it USED to have)... BUT for anything like that to work, you will have to go AROUND the stain, otherwise it will darken also, and still be darker than the surrounding leather. I wouldnt worry about it really. It is in a hidden area, it doesnt effect the function or soundness of the saddle.
Mildew and mildew is whitish and sometimes even has a green tint, AND it have a rough texture. You can scrape it off with a fingernail. Mold will sometimes be black. But it still has a grimy rough texture.
Angels are people who can see and feel the pain in all creatures... -myself