Creating a dynamic alpha based on cursor/object position

Hello!

I am wondering if it would be possible to create an alpha for an object that is painted real time based on where a users cursor or an object they are dragging/controlling goes on the screen. Basically the premises is having the player mow the lawn with a push mower and where the mower goes over one type of grass it changes to a freshly mown type of grass. Any thoughts on how this may be accomplished?

One thought I had was to create a bunch of small planes on the screen, each with their own grass texture and then just change the texture of the tile as the mower goes over it. However this is a very graphically intense way unless big squares are used and then it looks rather clunky and untrue to where the user may even be mowing.

Any thoughts on this would be greatly appreciated. Thanks!