Try to wipe it off with a lightly moistened paper towel. Make sure it is not wet, just moist. If it is too wet, it can take off the egg's bloom, a coat of gloss that protects the egg from bacteria. If you still can't get it off, try adding a little bit of salt on the paper towel and scrub gently.
Also, to prevent the loss of anymore eggs, I suggest that you separate her from the flock into her own box nest.
~Rio