High Sierra screen artifacts

I'm seeing residual screen artifacts on my desktop and also in application windows that were previously active, but which have been hidden or closed. These artifacts are very faint, and I think over a suitable period of time, the initial set of artifacts "fade"... but they're replaced by artifacts from other open then hidden/closed windows.

Also, if I change the desktop to any solid color, I cannot see these artifacts. And they are visible only with certain background images. (I think if the background image being used is very bright or very dark then you cannot see these artifacts.

NOTE - the artifacts are VERY faint - I've attempted to create some screen shots, which as I look at the screen are relatively easy to see, but when I look at the actual screen shots, they are almost impossible to detect.