- You might be looking for the episode "Unity".
The Unity was an alien spore that landed in Smallville.
Unity was an alien parasite that had managed to consume several worlds before landing on Earth. Its first victim on the planet, reverend Amos Howell, was sent to "convert" the people of Smallville. In less than no time, all citizens of the town were at his disposal. With tentacles, Unity tapped into the people's energy, becoming one with them. It planted a parasitic worm in people's brains, feeding off the electrical impulses in their nervous system.
With the people of Smallville under his control, it was his plan to send them on "missionary" work around the world to let the entire planet's population become one with the Unity.
Fighting off several agents of Unity, Superman and Supergirl discovered its weakness: x-rays. Using their x-ray vision, they successfully separated Ma Kent and a couple of doctors from the spore's control. They made their way to Smallville, where they separated more victims.
The Unity had left its base to merge with Howell, who had by now transformed into a being like Unity. Supergirl prevented the fusion, and set the creature ablaze with a gas canister. As Superman fought Howell, Unity slowly burned to death.