I'm appalled that the bulk of the replies have been "yeah your grandfather is wrong, but you should do something about your dandruff".
If the OP had long hair that bothered his grandfather, and the grandfather crept in and cut it off during the night, would you all say "yeah that was wrong, but you should have cut your hair". Or if the OP had a favourite t-shirt that the grandfather didn't like, and he threw it away while the OP slept, would you all say "yeah that was wrong, but you should have got rid of the t-shirt already".
I'm not saying whether it's right or wrong of the OP to retain his dandruff (I have a mild case and damn right I use anti-dandruff shampoo), but if he's okay with it then that's good enough for me.