This may have been a British show, but I could be wrong. I remember an old guy who was apparently an extra-terrestrial. He taught some kids how to fight these seaweed-like monsters. If I’m remembering correctly, they did it by concentrating on light, and a line of white light would come out of their foreheads and fend of the monsters.

I remember a scene where they were in some sort of cave under the ocean, and they could escape by jumping into the ceiling, which was kind of quicksandy (don’t ask me how), and they would submerge from the bottom of the ocean floor and swim to shore.

In the show finale, the old guy died, and his body got zapped into space in front of the kids…