Android how to use swipe to make certain part of an object fade out?

Hi guys,

I'm working on a small game for android. To make it easy there is a big cube in the center of the screen with a material applied to it. What I want (but I don't know how to do it) is that when the player swipe ( I know the swipe code) a certain part of that cube only that part start to fade out (reduce alpha).

So far my code make the entire object fade out no matter in which part the player is touching it.

Can someone help me?

Thank you, Diego.