Fridge Horror: There is a lot of it in the movie: the ice cream man who appears to be eaten to shreds in the flick was far from the first person to be transported there and never to return to Earth, the lizard men who fell into the volcano were under mind control at the time and may have been good monsters, and so on. Just watch the flick.
The very first scenes of the movie feature a lost astronaut attempting to contact Mission Control just before he's eaten by Grumpy. Later in the movie Rick Marshall sees an empty spacesuit—but not the astronaut who was wearing it—in Grumpy's nest. How did Grumpy eat the astronaut and leave the suit behind???
He has a bizarre digestion system, from what we've seen. It's possible that he just processed the astronaut and not the suit.
The suit is mostly made of rubber, plastic, and silicates. It would be more surprising if Grumpy had been able to digest it.