Paul's audience would have understood that this Psalm refers to the Messiah. Alternate translation: "In another Psalm of David, he also says about the Messiah" (See: [[rc://en/ta/man/translate/figs-explicit]])
The phrase "see decay" is a metonym for "decay." Alternate translation: "You will not allow the body of your Holy One to rot" (See: [[rc://en/ta/man/translate/figs-metonymy]])