Mike Daneen vs. Ted Gunther

The IMDB cast list gives Mike Daneen as the actor who plays Dr. Stein. When I watched this film last week, I was sure that the actor who plays Dr. Stein is Ted Gunther, who played Dr. Halvorsen in THE GREEN SLIME. Yet Gunther's name is nowhere to be found in the credits. I googled Mike Daneen and found a picture of him from TORA! TORA! TORA! (1970) that didn't look anything like the actor who played Dr. Stein. So why is Daneen listed and not Gunther?

Also, as soon as the spaceship carrying the main cast members, including Dr. Stein, gets back to Earth after their first mission, Dr. Stein disappears from the film--permanently--with no mention of why he's gone.