The Dragon Flame Seed boosts all energy with a Yang/Flame effect. Even divine energy becomes hot and fiery as if it was related to Flames by birth.
Therefore, the moon energy which is Yin and Cold in nature it will become hot once stored in the second energy field that has an affixed and permanent Flame Dragon Seed.
The Flame dragon seed couldn't be destroyed, as it would fight back and in turn, destroy Feng Mei's physical body or at least cripple him.
It's all because of the pride of a dragon.
Even though Feng is its creator and It is still in its nascent phase, it still won't allow Feng to end it. It will fight back with everything it has, no matter what, to remain alive, even if it means killing its creator with itself.
Anyways, Feng wasn't gonna destroy it from the very beginning. The Flame Dragon Seed was one of the reasons why he could end Tian Wu's heavenly realm in one move.