In Japan Home or homeware shops, they sell this white sponge that takes off everything gently. It works like a very very fine buffing sponge that you use to polish/shine your fingernails. You just slice off a bit of the sponge, wet it then gently rub it on the wall. Sorry I don't have the name as it is in Japanese but it is widely sold there. I swear by it & I use it all the time when an odd marker or scratch leave a mark our walls.
This sounds like another version of the Magic Eraser that mentioned as it works the same way - so it may be more widely available in HK than I thought, which is good to know.