is this a new monitor?
how big is this 'grey spot'? (or is it the colours or detail in that area are less vivid)
has it just appeared or been there a long time, or since you had the monitor?
if, from what you have described, that the problem is gone when you move to the side of the monitor, there may have been a manufacturing problem with the monitor, where the layers of filters applied to the actual screen were not done properly.
i had a monitor like this but it was 100% clear that there was an issue from the start, you could sit centre onto the monitor and you would just see a greyed/blacked out image with minimal detail, move to either side and it was clear, as it should be if you look strait onto it. was a faulty monitor.