Simple: They would move sideways. If everybody just invested into indexes and ETFs, for those particular market days, the stocks move slowly.
Some stocks may disregard the market and still move as normal, but the majority would perform slower for the day, or be extremely volatile for a small price range (e.g. consistently go up and back down and not really in any particular direction).
Index funds and ETF's are becoming so prevalent. What if everyone started passive investing? How would individual stock prices even move?
