Yellow arm glow marking invulnerable crawlers, which fails Carl mid-rescue when Eris cancels it.
Carl and Dong Quixote are the only two of the group with immortality, which is why they push into the Surgical Theater alone while the pets stay behind. Carl double-checks the yellow glow on his arms before opening the double doors and notes that this version isn't as good as the one he had earlier — he's pretty sure he'd survive the Sheol Fire, but not positive, and Burcu has none at all and would absolutely be turned to ash.
Mid-rescue, Rosetta reports that Eris has woken and removed everyone's invulnerability. Carl looks down and the yellow glow is gone. Seconds later the table tips, his bare feet hit the floor, and only Gloom Wraith Phase keeps him alive. Dong's own yellow glow is visible on his arms when Carl orders him to kill Architect Houston by stepping into the room.
A yellow glow on the forearms of an invulnerable crawler.
The protection that lets Carl assault the theater, and whose loss nearly kills him.