Is there an open source derivative of this?

Genrally speaking, https://alternativeto.net has answers. Not for "Sheet Music Scanner" literally though. Feel free to broaden your search.

edit: sorry, can't help bickering: Derivative would mean that the app must be derived from this specific iOS app. Since the concept of the app is not unique by a far shot, you probably mean alternative. There are extremely few actual foss derivatives of anything, although the Linux kernel itself could be described this way, at least historically.