There has been a long held belief that if you stop exercising after building muscle, your mass will turn to fat.
This is a myth.
Muscles and fats are made from different cells so they have little or no correlation. Muscles are made of long thin fibres that cannot physically alter into fat cells.
The simple science is: exercise and training increases muscle mass (hypertrophy) and not working out decreases their size (atrophy). In contrast, globular fat cells grow and shrink dependant on diet.
If you do put on weight after stopping a muscle building regime, it is most likely because your calorie intake is too high for a non-working out regime. Not because muscle is turning into fat.