I think something like this happened to me last night with an older SE. I am not sure what I did. I was, I think, switching between mail and the home screen and then maybe back to mail. My mail icon was shaded on the home screen.
The shade is the change you get when you press down on an icon. You can test by touching an icon and looking around your finger to see it shaded. For icons in the dock you can press down, keep your finger down but drag it somewhere else on the screen.
This is obviously supposed to change back to normal when you lift off the icon. That didn’t happen. So there might be some bugs with iOS 11 related to user interaction and tapping icons.
To fix I think I just closed mail and tapped the icon again.