I remember in high school we asked our teacher if a boy who looks “at risk” says good shabbos to us is it a chillul Hashem not to answer back, because it might turn him off more that Jews are “impolite” etc…. She answered NO WAY
But my personal opinion is that if its just a “no pause, keep walking” kind of good shabbos than i think its ok. But if he looks like he is interested in continuing conversation, than no.