Meaning of もったいないお化けがでる

If you don't finish your plate, then the leftover food will come to life and this you call もったいないお化け.

Apparently, this concept originates in this TV spot and as far as I can tell is also nowadays a common way to encourage children to finish their plate.

Telling children that their leftover food will turn into monsters if they don't eat it may sound quite bizarre when you say it like this, but I guess in the context of Japanese culture it is not such an outlandish idea, considering that objects may have a kind of "soul".