Thank you VikingOSX,
Both icons, in the Dock and the Applications folder are affected. And Pages is the only application with the generic icon.
Making a copy of the Pages application produced a perfect Pages icon, both in the Applications folder and the Dock.
BUT, it has the word "copy" in the name: Pages copy. app.
Renaming it to "Pages.app" reverts the name back to the generic version.
Booting into Safe Boot Mode multiple times makes no difference, it's still the same white icon.
Another suggestion that did not work was to reset launch services, that also failed.
Pages still works as expected, it just doesn't have the proper icon, and it's a perplexing problem.
After making the copy of Pages I foolishly deleted the original Pages.app.
Then I made another mistake by changing the name of the Pages copy.app to Pages.app.
Now making another copy won't work.
That's why I asked if the Pages.app can be deleted from the iMac and re-downloaded from the App Store.