The green slime creatures were played by Japanese children in bulky monster suits.
Many of the background players are American military personnel who were based in Japan at the time.
One of the few credited screenplays for comic book writer Bill Finger, who co-created Batman with Bob Kane.
The original storyline for Monster aus dem All (1968) originated in Italy, where MGM also had dealings. Years before The Green Slime went into production, MGM had contracted Italian filmmaker Antonio Margheriti to direct what was originally intended to be a series of four television movies about the adventures of a space station called Gamma One. Margheriti's films in the series consisted of Raumschiff Alpha (1966), Tödliche Nebel (1966), Orion 3000 - Raumfahrt des Grauens (1966) and Dämonen aus dem All (1967), all created over a period of three months and released in 1965. MGM was impressed with Margheriti's films and released the four films theatrically. Gamma One producers Walter Manley and Ivan Reiner were eager to take advantage of these films and made The Green Slime as an unofficial fifth entry in the film series. The only connection the film has to Margheriti's films is the space station, re-titled Gamma Three, which has a similar design as the one in Margheriti's films.
Charles Fox, who wrote the theme song for this film, also co-wrote the songs for another sci/fi flick of 1968, Barbarella (1968).