I understand that you are concerned about your kitty's possible exposure to Efudex (fluorouracil, also known as 5-FU).
Ingestion in dogs and cats generally leads to clinical signs within a few hours. Initial signs include severe vomiting, which may progress to bloody vomiting and diarrhea, and then can progress to severe tremors, incoordination, and seizures.
This drug may be converted to fluorocitrate and interfere with the Kreb's cycle, which may be one cause of seizures and ataxia. It destroys rapidly dividing cells, affecting the gastrointestinal tract, liver, kidneys, central nervous system, and bone marrow. The mortality rate among animals ingesting 5-FU is high.
So at this point you want to get any possible drug residue off your kitty's fur.
Use a solution of Dawn dishwashing detergent (or any degreasing type detergent) and warm water and clean his fur on his forehead.
If you remove the product then there should not be a chance of him ingesting it.
Then you need to watch him closely for any signs of stomach, which would be the first sign of toxicity. If he seems nauseous or has diarrhea or vomiting he should be seen by a veterinarian on an emergency basis for treatment.
Please let me know if you have any further questions.