Some of mine do that too, I offer them a piece of stone salt to lick from time to time, but they pretty much ignore it, they need so little that I wouldn't notice anything missing.
I'm not sure why they eat plaster from the wall. I try to feed a diverse diet, so they should have everything they need. I don't think it's something to worry, as long there's no toxic paint (like old lead paint). Since I can't stop them, I stopped worrying, they nibble on plants I wouldn't feed them like ivy with no ill effects, so I trust them to know what they are doing.
It could be that there's something missing in their diet that they are craving, but I wouldn't know what it is.