No matter if you are on Earth, the moon or just chilling in space, your mass does not change. But your weight depends on the gravity force; you would weigh less on the moon than on Earth, and in space you would weigh almost nothing at all. Where ever you are, your mass will always stay the same.
Mass never changes, and is constant everywhere. The weight is what changes. If you weigh 50 kg on Earth, you'll weigh 50 kg everywhere else in the universe. That's because the amount of matter in you cannot change.
