Dead bodies decompose, and thus gather moss. However, there is an ambiguity. Does "a Rolling Stone" mean "any rolling stone"? In this case, clearly all members of the Rolling Stones, (including Jagger) are alive (or else cremated). However, if we simply mean "there exists a Rolling Stone, which gathers no moss," then we simply mean that not all the Rolling Stones are dead, though Jagger may be.