what is this blue spot on my Macbook Pro?

I changed my screen cover this morning and i come back to see this. What causes it? It is internal and when i shut down my screen it's gone. It only shows when the back ground is dark. My friend said that my screen affect the magnet inside but what is it really? How can i fix that? will it go away over time? or was it the pressure i press on my screen? Thank you Please help!