I reckon its one out of two things:
1. There is a magnetic force near your eMac. Try moving the eMac to different rooms in your house and see if the pink spot changes.
2. Your CRT is dying. You'll sometimes get bits of the CRT go pink or green when the CRT is dying Although sometimes it just goes away on its own. Like my 13 year old TV: A few years ago it got pink around the edges and I thought it was on its way out... But in a couple of weeks it came right again!
Good luck!