There are several types of ficus; none of them would be recommended for ingestion by a cat, and are potentially poisonous. You would need to watch for side effects of drooling, loss of balance, lethargy/depression, pain, problems using the litter box or anything else that you sense is 'off' .
Again, the level of concern would depend on the type of ficus and even which part the cat ate (as well as how much).
When in any doubt - a vet visit wins out!
For a list of poisonous plants (offered thru UC Davis) go here