This is probably due to an automated ship, like a probe and sometimes a survey ship, targeting a spot in the fog of war for its next destination. When it gets there, it finds a planet. It doesn't have population, so it can't colonize it. You can ignore this, and the ship will go somewhere else.
If it continues, you may have manually told a ship that isn't a colony ship to go to a spot in the fog of war that contains a planet. When the ship moved close enough to see the planet, it stopped and sent you the message. I'm not sure if the ship will cancel its movement order and wait for a new order from you or continue trying to move to the planet and continue sending you the message. You will have to do what is necessary to find the ship and send it somewhere else.
