Before Little Hiding Dragon grows and metamorphoses to the Seventh-rank, its appearance still won't be fully developed. Moreover, Little Hiding Dragon has the Dragon Transformation Technique, so Chu Mu didn't worry about anyone recognizing it as the Azure Sky Hidden Dragon before it reached the Seventh-rank.
Most importantly, since Chu Mu forcefully added an attribute to Little Hiding Dragon, its body would undergo a certain degree of change at each stage of metamorphosis. Perhaps after reaching the Seventh-rank, Little Hiding Dragon would have transformed into a Phantom Azure Thunder Dragon, and its shape would naturally be different from the Qingzhe Dragon.
The identity of the Qingzhe Dragon is too sensitive and easily exposes it, so deciding to add the ghost attribute to Little Hiding Dragon is also to cover up its characteristics.