Here being in slavery to decay is a metaphor for being certain to decay. Alternate translation: "from being like a slave to decay" (See: [[rc://en/ta/man/jit/figs-metaphor]])
"Freedom" here is in contrast with slavery to decay. It is a metaphor meaning that the creation will not decay. Alternate translation: "that it will become gloriously free from decay like the children of God" (See: [[rc://en/ta/man/jit/figs-metaphor]])