This recipe uses frozen corn kernels. You can of course use corn from grilled corn on the cob. Usually I don't have any leftover corn when I grill corn, and if I do grill it, I want to eat it right then and there, on the cob. Mexican crema is a lot like runny sour cream. I don't usually have it around, so I usually use diluted sour cream when a recipe like this one calls for crema. But if you have crema, use it!