Shouldn't this be a spell, instead of a permanent? Permanents (when they come onto the field) aren't really able to target anything from what I recall, unless it requires a set-and-fire trigger where you wait a turn after setting it down (thus making it vulnerable to PC, like all the other Time permanents...), and then target a creature?
I would find it more useful as a spell that places a special icon (not the
one, which is reserved for immortality) on the creature, denoting that no more 'copies' of that card can be produced (or mutated into).