Why do I have grey spots on my iPod screen?

I recently had my iPod screen replaced by my friend. The glass didn't lay down completely flat and I thought it would be fine. There was a small 1-3 mm gap. A few days ago I was outside in my backyard and noticed a grey spot appeared in the top right corner of the screen covering the battery icon. The next day it had moved down and changed form. Whenever I try to put pressure onto it to move it stays still. Also it seems like it's being affected by gravity. What do I do? I don't want to replace my screen again.