rock plant vs plant life

rock plant and plant life both are nouns.

As nouns, plant life is a hypernym of rock plant; that is, plant life is a word with a broader meaning than rock plant:

rock plant: plant that grows on or among rocks or is suitable for a rock garden
plant life: (botany) a living organism lacking the power of locomotion

Other hypernyms of rock plant include plant, flora.

rock plant (noun)	plant life (noun)
plant that grows on or among rocks or is suitable for a rock garden	(botany) a living organism lacking the power of locomotion