The control effect applies so long as at least one Aura is attached to the creature, and will continue to apply even if Rootwater Matriarch has left the battlefield. The effect ends if at any time the creature has no Auras on it.

Unlike many similar creatures, Rootwater Matriarch untaps as normal during your untap step. Whether it’s tapped or untapped has no bearing on the control-change effect.

A creature is “enchanted” if it has any Auras attached to it.

Can target an unenchanted creature but has no effect on that creature.