Hmmm Hops.
How to tell the difference between manipulative creepy charm and real grace?
Well I wouldn't call 'charm' 'grace'. So, charm...
The power or quality of pleasing or delighting; attractiveness
A particular quality that attracts; a delightful characteristic
and then we get the 'magical' associations (is this coincidence?) such as: an action or formula thought to have magical power.
I can see where I might use 'charm' to describe something other than an adult in a positive way. But to say that an adult is charming or has charm, I'd tend to be using it in a derogatory manner, meaning that they are manipulative or
superficial. Okay, nothing wrong with being superficial -
or is there? There is, to me. So: I don't rate charm at all.
Real grace is a totally different thing, to me. Tutu - charming?
'Charming' is weaving a magic, a manipulation. If I was 'charming', I'd be acting towards an objective. This might be a positive objective, but it still wouldn't be 'me' being 'me'.
Grace is someone 'being'. So much bigger and so different a concept that I can't start to expand more.

As for how it works with Nism, I reckon their particular charms work with a certain sort of person. And on a big scale, it changes and gets called 'charisma'. Bill Clinton comes to mind. Is (was) he charming, and/or does (did) he have charisma? And for whom?
I doubt there was any charm or charisma in my family. Or I just saw right through any attempts at it.