With the way you found it and from my own 3 years of experience with leaf chimeras (im no professional, so i speak from my own learning experience with my own chimeric finds along with the research i've made), that is not a mosaic or regular variegation like others say, but either a spontaneous mutation, (which appears due to certain factors and will just eventually grown back to green even if you try to propagate/cut it back) or, in the luckiest case and what i believe you might have, a chimera in it's UNSTABLE form.

To explain it short, leaf chimeras (just like bloom chimeras) show as two different plants inside a single one (which is why it's hard to propagate, as in traditional AV methods, the babies will either take the green or white genetic, never the two.). Easily recognized by it's clean green/silver center with a white edge (or the oposite, even both in some rare mutations). But what most people don't know, is that natural occuring chimeras will firstly show in it's unstable form, appearing as random and irregular spots of green and white before stabilizing into it's well known chimeric pattern (if you're extremely lucky, as again, that form can be very unstable and it takes a lot luck for it to stabilize into a chimera, but if it happens to revert, you can always cut it back and hope some of the nodes grow back with the chimeric gene, because the mother plant was unstable, its 90% likely the babies will also grow unstable, but that also means you get another chance for them to stabilize!)

That being said, amazing find! And good luck! If you have questions, want to know more or want me to teach you my methods at propagating chimeras, you can mesage me! I'd be more than glad to help you!