Even if he knew about it, it's unclear whether the emergence of a new Celestial would have been a problem for Ego's plans.
As explained in The Eternals, the unborn Celestial draws energy from the life living on Earth.
Ajak: Five years ago, Thanos erased half of the population of the universe. But the people of this planet brought everyone back with the snap of a finger. The sudden return of the population provided the necessary energy for the Emergence to begin.
Ikaris: How long do we have?
Ajak: Seven days.
So first of all, given that Ego presumably wasn't planning for the Blip, he probably would have suspected he had longer before the Emergence. In addition the process of terraforming would have killed all life on Earth (and all the other planets he seeded), cutting off the Celestial's food source. The Celestial clearly wasn't immortal either, so it's entirely possible it would have been killed by the process too.