Muscles can't possibly turn to fat. When you exercise, your muscles become larger and stronger because exercise causes extra protein building blocks, called amino acids, to deposit in muscles. All day long, amino acids pass from your muscles into your bloodstream and then back into muscles. Exercise is the major stimulus to force amino acids back into muscles.
When you stop exercising, fewer amino acids go into your muscles so the muscles get smaller. Your body has no way to store extra protein, so amino acids that are not used in your muscles are picked up by your liver, which uses them for energy or converts them into fat for storage. So if you stop exercising, you have to eat less or you will gain weight. But muscles never turn into fat.