No, that is not how you would get exposed to rabies. You would need to be bitten by an animal that has rabies.
Rabies virus dies very fast outside of the body, as soon as the saliva dries or even before it dries. I wouldn't be scared of the floor, I would be more scared if your Grandfather sneezed in your eyes or into your open wounds, if he has it. And if he had it he wouldnt show symptom until 8 months in, and by the time they show symptoms, it is too late for them. And would leave you very little time to get the multiple rabies vaccine shots to have a high enough chance of survival. But I wouldnt worry about it, if he has been doing this for a long time, span of more than 2-3 years. If it is a new thing he is doing, and you have visibly seen rabbid animals around town more than once, then I would move away from him, if I could.
Ask him if he is willing to get the rabies vaccines, because if he does get the Vaccines he will be immune for around 5-10 years, and safe to be around again.