What happens if I wipe the LCD surface of my HP-12C 1982 version with alcohol?

[hojeongko]

I picked up a 1982 USA production HP-12C today.
There was some dirt on the surface, so I wiped the surface, including the LCD, with alcohol.
It looked fine at first, but after a while it seemed like something was peeling off the LCD.
I tried to remove it using tape, and something like a thin film came out attached to the tape.
There are no problems when operating the calculator. There is no problem with the display on the LCD.
However, I am curious about what the thin film attached to the tape is.
What type of coating was it on?