Blue mark on my iPhone (4) s screen?

I can't take it to the apple store today because there's no genius appointments. I just want to know what it is. I got it yesterday as my other iphone 4s sleep/wake button broke. I can only see it on white things such as my emails, google and messages but its highly annoying. Its about 5mn in hight and 3mm in width.

Your iPhone screen probably developed a fault and needs to be replaced. I could recommend TFix Repairs located in London, you can send it in and they will diagnose what the problem is. The website is www.tfixrepairs.com