if its a small frag... no.. it will probably die somewhere and become part of your aquscape, or it will somehow come back with a vengence and take over your entire tank, if its like a Xenia, GSP, Zoa.
Will it polute your tank if it dies?
Depends on how small it is, and how large the tank is.
Seeing how you LOST it, id say it will probably not even put a dent in your chemistry, because if was something you can not lose, id say that would be more of a problem with chemistry, or if it was a Anem.
So cliff's... if its a Zoa, GSP, or Xenia... id look for it, or your gonna get sad with it spreads crazy.
If its something like a tiny euphylia, blastos, acan, or a SPS, i wouldnt sweat it, however id probably blame an urchin if u have one and look for it, as its probably taxing it around your tank with it.