It was called molekiller. With moles that travel vertically from bottom to top of the screen and a lawn mower that goes horizontally from left to right of the screen; and sometimes gets a mole. After a while the whole screen is bloody red and messy. I know it's a bit sick, but I only used it for a short time.
I'm also looking for links to other screensavers that fill up the screen gradually, like the above one.