If I flash in Teferi, Time Raveler, can its passive ability counter a spell on the stack?
Your suspicion is correct, Teferi prevents opponent's cards from being cast outside the main phases, but if the opponent's spell is already on the stack, then it has already been cast, and as such Teferi's passive will not have any effect on it. They could not, however, respond to the stack again after Teferi resolves.