If nature blew schach on my sukkah, do I have a problem of ta’aseh velo min ha-asui?
Sechach that falls on one’s Sukkah is not considered kosher sechach, because of the problem of taaseh velo min ha-asui.
However, if it is only a small volume of leaves, and it is mixed in with your sechach, it will not constitute a problem, because the majority of the sechach is kosher.
To avoid the problem altogether, you should slightly move the sechach (e.g. raising it up and allowing it to drop) with the intent for the Succah to be kosher. This will render the fallen leaves part of the kosher sechach.