Anew weight-loss fad claims that you can reduce your mass simply by traveling to the moon where gravity is weaker. why is this certainly a scam?

Your mass can never change no matter where you are since mass is the amount of matter you contain.  By going to the moon, you do become lighter due to the weaker gravity but the amount of matter that you are made of (your mass) does not change.

I hope the helps.  Let me know if anything is unclear.