CRT screens are a phosphor material coating the inside of the screen
electrons fired from the cathode end of the tube impinge on the phosphor, causing it to glow
after a while, as you observed, some of the phosphor is burned off, and as in your case,
a "message" gets burned into the screen
the original personal computers used screen savers so that the electron beam did not
impinge on the same spot all the time, saving them from the problem you describe.
it wasn't too much of an issue with televisions, because they never had the same pattern all the time
it was a problem with any CRT - oscilloscopes and radar screen and the like, because they could be
showing the same thing for along time over years.
I do not think its repairable.
You might be able to replace the tube with another tube, (probably a salvage from a similar device) or with a modern flat screen.