First of all, none of this poison butterfly bullshit was ever mentioned in the show, therefore it is not Game of Thrones canon and cannot be said to be a factor in Grey Worm’s future.
Secondly, while Missandei was taken from her homeland at a very young age, if these evil insects are a part of the reality of the show, wouldn’t she have known about it and mentioned to GW at some point that her home is known as the Isle of Butterflies and oh, yeah, don’t ever go there ’cause one of them will kill you dead right quick?