The pinched-fingers emoji (a hand with all five fingers pinched together) originated as the Italian gesture 'che vuoi?' ('what do you want?' or 'what are you saying?'), expressing exasperation or 'come on'. Online it is widely used to mean 'a small amount', 'precise / perfect', or to add Italian-meme flavour ('mamma mia'). If your teen uses it, they are most likely doing Italian-meme humour or signalling precision. Harmless.