Shen Mu pressed his lips together which spoke of the myriad of emotions brewing within him. There was relief and joy from finally realizing his mother was alive. Then there was the surprise when he realized his mother was a domain expert, not only that but a gold-grade craftsman too, and from the Divine Armament City no less. Even though she wasn't a member of the Divine Armament Sect, her identity as one of the craftsmen in the City it controlled and being a gold-grade craftsman to boot, already showed how powerful her identity was.
That realization unearthed other emotions in him which boiled down to frustration and anger. The explanation she had given on why she couldn't help his father made sense and even some part of him was relieved she did so, at least because of it, he still had one parent alive rather than risk losing them both.