If you don’t plan on eating the entire fettuccine alfredo recipe, I recommend refrigerating leftover sauce before it has been tossed with the noodles. It will keep up to 5 days in an airtight container.
To reheat the sauce, the best method is on the stovetop. Place it in a saucepan (heavy-bottom triple-ply stainless works best) and reheat gently, over low heat, whisking frequently. If the alfredo has been mixed with the noodles, there are several recommended ways, such as heating with steam (using a double-broiler), in the oven, or using a microwave.