The story is that during the Apollo mission, the astronauts switched to the medical channel and reported 'they're here, they're on the crater, and they can see us,' referring to extraterrestrial beings.
This is a piece of UFO folklore originating from a 1969 tabloid fabrication, with no supporting evidence in actual NASA transcripts.
The story traces back to a 'Pepper Transcript' published in the National Bulletin tabloid (September 1969) and later amplified by UFO author Otto Binder. Versions of the alleged quote vary widely (e.g., 'They are standing along the side of the crater... they are observing us'), but none appear in actual Apollo mission transcripts. Investigator James Oberg documented multiple internal errors in the alleged transcript (wrong NASA terminology, fictional technical jargon) and noted that up to 1,500 journalists monitored NASA communications live with no irregularities reported. The 'medical channel' suppression mechanism has also been refuted by NASA officials.