iMac Screen Burn In?

I've had my iMac (Retina 5K, 27-inch, Late 2014) for years but lately I seem to be seeing alot of what I can only describe as screen 'burn in' but not fully sure if it is. This also only appears to be happening on the perimeter of the screen as you can see below, the 'burn in' is stronger on the right hand side which is closer to the edge of the screen. I would of expected a burn in to be fully solid. It stays for an hour or so and then disappears. I have the same issue with my dock and menu bar but never in the centre of the screen for eg, just the outer edges.

I'd love to hear from anyone who knows what's happening here and why I am only seeing this now after having the iMac for years and never had this happen before.