HP 48G screen is kind of cracked

[BruceH]

That's very unusual - never seen anything like that before. It looks like whatever it is is on the screen's surface so two possibilities come to mind.

One is that a previous owner attached a screen protector (perhaps by trimming down a mobile phone one) and that has now 'gone bad', in which case you might try to see if an edge can be peeled up.

Otherwise something may have been spilt on the screen - but given how clean the rest of the machine looks around and under the bezel, that seems unlikely. Maybe start by gently rubbing with one of those spectacles cleaning wipes?

I certainly wouldn't use sandpaper - if only because you will inevitably scratch the bezel. And if you do use anything liquid, remember to hold the calc upside down so an inadvertent spill runs down your hand rather than into the calc.