Moon was a softer geno art. To create light, you could not use pure power.
When Yisha gave Moon to Han Sen, she hadn't shown him how to perform it. Han Sen believed this was because she herself was not well-versed with Moon, and even if she knew how to perform it, she couldn't do it spectacularly well.
That was because Yisha's power was too brutal. It lacked a softer finesse that was required. A geno art like Moon needed a gentler touch, and therefore, Yisha could not learn it.
Han Sen had wondered why she wanted him to learn it. He hadn't shown her his gentle power, and Yisha didn't excel at it herself. But even so, she had given him Moon to learn.
After much deliberation, Han Sen was able to understand the reason.
Yisha was still suspicious of him. It hadn't gone away after Han Sen displayed his talents in the battle for Red Mist Valley. Moon might very well be another test for him.
