Leftovers should never be at room temperature for more than 2 hours. They should be refrigerated quickly. Refrigerators are designed to accommodate changes in temperatures. Though the plastic wrap may prevent contact with other food and bacteria, it will not prevent growth of bacteria already in or on the food if left out at room temperature. Ground beef is risky to eat rare because surface bacteria are transferred to the interior of the meat during grinding, giving them a much greater surface area on which to grow.
Mold is result of spoilage.
If there is mold on hard cheese, cut off the mold to a depth of one inch, and it should be fine to eat. Other foods with mold on them should be thrown out. The food is likely spoiled. It's not worth the risk of becoming ill. Bacteria are everywhere - on every surface that isn't sterile, in our bodies, in animal's bodies.