Nacho cheese does go bad if not refrigerated within 2 hours of opening the can cheese.
If you have nacho cheese in a can you can leave the nacho cheese out at room temperature for up to 2 hours before it goes bad.
If you've left nacho cheese out longer than a few hours then it can be bad because of bacteria that can grow on the nacho cheese.
So make sure to refrigerate any leftover nacho cheese after opening it within 2 hours.
All perishable foods including cheese is safe to leave out at room temperature for up to 2 hours and after 2 hours the food should be thrown out for safety.
Although you are at eating the cheese or other perishable food at your own risk if you left it out at room temperature longer than a few hours.
I'll admit that I have before eaten nacho cheese left out at room temperature longer than a few hours and never got sick from it.
But there's still the chance that you could get sick from it.