thanks for your suggestion, C_P. I just went back to Pshop,
and the problem seems to have vanished on its own.
I recall this same thing happened a couple months back; both times on an image that I found on "google image search" on the internet. Odd because that "56px" overlay in a pink square was showing on all 5 images I had open in photoshop, and when I used the file-open dialog, it also appeared super-imposed over the preview of the file-open image. I wondered if it was some kind of watermark applied by image copyright holders.
the only thing i can think of is some kind of software bug with this version of Pshop + Mac Mojave.
If it happens again try to see if turning off »Use Graphics Processor« in the Performance Preferences (Photoshop > Preferences > Performance > Graphic Processor Settings) and restarting Photoshop has any bearing on the issue.
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