I thought it was perhaps a leaf that had fallen as well. I'm not sure how long it has been developing and I'd only just noticed it, but it is attached to and growing from the surface of the larger leaf. My first (naive) guess was that maybe - just maybe it was a keiki though I've never heard of one growing on a leaf before.
I doubt very much that that small leaf is viable though, it's thin and limp and, as can be seen in the pic, beginning to pale already.
I just have not, in my short experience, ever seen anything like it before.
Thank you for the edit WhiteRabbit