When I was a kid I watched a black-and-white American SF movie.

A spaceship returned to Earth. Two astronauts died and one was charred but still alive. There was a plot point about doors of the capsule that were so hot the rescue team had to pour water on the door. The astronaut survived but died soon afterwards and scientists found a small starfish-like creature. It escaped and started to grow eventually as big as a building. Eventually they stopped the creature by electricity.