Is it there all the time. On different screens or does it dissappear sometimes? If its there all the time, the screen has probably got damaged in some way and the pixels are not working in that spot. If so it will need replacing. Without seeing it, difficult to say.
If you go into the phone app and tap in *#0*# You will see options to test the screen colours. If the area where you have a problem is still there, then you have dead pixels in the screen and need a replacement screen. If that's the case then go into the support section on the Members App and speak to the customer support team to get it looked at. The heating issue depends on how bad it is. You could try clearing the Partition Cache on the phone. Usually solves any problems after an update