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If the snot is green or yellow, you should be concerned. If not, it could be allergies or a "cold" and the cat will recover with time.
If the mucus is green or yellow, it indicates that that there is an infection present and this is not good. Cats need antibiotics to recover in this case.
Until you can see the vet, you can use pediatric (not adult) nasal saline drops in each nostril to clear the mucus. You might also want to warm her food since she cannot smell it.
Let me know if you still have questions.