Problem in Ultra Key

Hi, 

I am using Ultra Key to remove the green screen (in my case, I am using a white screen). This might be an idiotic question but for some reason, when I drop the eyedropper tool to choose background colour, a white box appears in my video. It stays there even after I export it. 

When I increase transperancy, under matte generation, it completely takes over. If I increase transperancy to 100, the only thing I see is a white box with a black frame. In the posted picture, you see it at maximum transperancy. What is this, and how do I remove it? 

Thank you!