The earth is motionless in the geocentric model. A planet, by definition, "wanders" or moves. The great and holy fathers of antiquity, who all taught that the earth is at rest, in the center of the universe, never called the earth a planet; however, fathers such as St. Basil the Great and St. John of Damascus do refer to the sun as a planet.
Definition of planet
https://en.wikipedia.org/wiki/Definition_of_planet
Referring to the earth as a planet is an innovation of recent centuries adapted to popular belief in the heliocentric model, where the earth is purported to rotate on its axis while orbiting the sun.
It is ill-advised for Orthodox Christians to refer to the earth as a planet, since this implies the earth is in motion, when it is not.