You don't need to save moon coins. In fact you can spend them all as soon as you get them. However you need to earn more moon coins than the limit you see in daily practice and anything that exceeds the limit turns into strange stones. Usually it is around 45k on Mondays and increases to 64k on Sundays. It is hard to reach at the start of the week but if you do all dailies that give you moon coins and some bonds you could reach the limit on the weekend.