What the four Treant Elders referred to as Ancestor Fu, is precisely Mu Yanfu, who is the actual elder brother of Mu Yanshan.
Back when the Saintess of Mirror Moon Palace was recruiting a husband, Mu Yanshan led the Treant Race team and made a move against Chen Tang after the recruitment ended, but was ultimately crushed by Chen Tang.
This incident alarmed Mu Yanfu, who personally arrived but was blocked by Mirror Moon Palace Master Wen Jiyue. The two battled into the Starry Sky, and it lasted for half a month before it ended.
Since both were higher Thirteenth Stage, neither could defeat the other, and thus the battle fell into ambiguity without an outcome.
However, Mu Yanfu did not leave the Great Kyushu Realm. This Treant Race team was summoned by his order, and from what the four Treant Elders said, they clearly intend to join forces with the Kyushu Qin Family to strike against the Other Shore Base...
"Rest assured about this..."
