In episode 4, Waleed (Farhan Akhtar) explains during his exposition dump that the Noor Dimension is a realm parallel to ours, and we are unable to see it as a veil separates it from ours. Explaining the Clandestine plan, Waleed says that if Kamala’s bangle is used to create the portal necessary for them to go home, it will only consume our world, and transform the entire terrain into that of Noor’s. While this explains why humans would be transformed/killed in the process of the Noor dimension swallowing up ours, it is a bit baffling as to why it kills the Clandestines, who hail from that dimension in the first place.
There’s a prevailing theory that the Clandestines and the Inhumans from the Marvel comics are one and the same, especially due to the fact that the mockup of the Noor dimension looks extremely similar to that of Attilan. Attilan is a Inhuman megacity that was shifted to the Himalayas by Black Bolt at some point in the comics, however, that connection in the context of “Ms. Marvel” is too vague to arrive at a logical conclusion (unless the show provides more context in the final episode).
The Clandestines in the show do not really possess special powers apart from standard superhuman strength and agility, so could it mean that their true Inhuman powers can only be awakened after they undergo Terrigenesis? The mist from the Veil does cocoon their bodies before killing them, which could mean that their bodies rejected the mutagenic process altogether, as dramatized in “Agents of S.H.I.E.L.D.” and the short-lived “Inhumans.”