There is no absolute standard for what a mikveh lady should say.

Often, she will say "kasher" or "kosher," to indicate the dunk has met all requirements. Sometimes a mikveh lady says "tehorah," to indicate all immersion requirements are complete. Other frequent comments are: "tizki lemitzvot," that you should merit to perform other mitzvot, or "(shenishma) besorot tovot," that we should hear good tidings.