When hit by sun rays, the moon creates excess energy, it increases the energy & reflects it back to earth as mana rays to make magic possible (the exact process is not important). Most of it quickly vanishes into the atmosphere if not used or stored. I imagine mana to work similar to light, but capable of being temporarily "absorbed" by matter like heat to make it linger on earth for practical reasons.
Magic users(about 5% of the population, but dependent on the nation), gods(not literally, but practically just immortal wizards in this context) & some animals & plants can store it or use it for spells, ranging from changing the environment to healing, creating magical artifacts, etc. Unless magic users receive extreme amount of schooling, they are only really capable of very limited feats like creating sparks or dubiously helpful lucky charms. It takes more than a lifetime of commitment to use more than the residual mana left inside ones body.
The (potential) problem
In the process of creating mana, the moon breaks the first law of thermodynamics. Gods (and very talented humans) can also imitate this process, although the amount is limited by the mana they can utilize(equivalent of 1 TWH for gods, 1MWH for humans), the meager results(10-20% increase) and the effort it takes(a day for maximum amount). The reverse is also possible under similar rules, but doesn't see much use. I also assume creating/destroying matter with energy is practically impossible to avoid the world filling up over time without adding new rules.
What possible problems could arise from such a system for breaking the laws of thermodynamics?