The redheaded performer gave a solo rendition of the Sigma song, "Find Me," before singing a duet to OneRepublic's "Apologize" with the band's frontman, Ryan Tedder himself.
Though both judges Lionel Richie and Luke Bryan warned Cassandra to watch her "nerves" on stage, Katy, 36, couldn't stop gushing about the Idol hopeful.
"Your voice is a spiritual experience," the "Firework" singer declared. "It is otherworldly angelic."
She went on to get candid about her grooming routine as a new mom to daughter Daisy.
"As a new mother, I haven't had much time, so I quit shaving my legs, but when you sang, the hair on my legs grew an inch and a half," Katy teased as her fellow judges put their heads in their hands. "Full body chills. It was amazing!"
She later tweeted a GIF of a pair of legs growing hair, writing, "An actual live look at my leg hair while I hear @casscoleman sing #AmericanIdol."