The Moon theme plays when you're searching for the treasure on the Moon. The title comes from the DuckTales: Remastered album, which matches the in-game title of the stage. Like the rest of the soundtrack, the music was originally mistakenly attributed to Yoshihiro Sakaguchi, before Tonomura was confirmed by Capcom audio personnel as the composer.
This song is noticeably longer than the other background music songs in the game, and it's style sounds more like it was intended to be the game's ending music rather than stage music. In fact, the game's ending uses the Stage Select music, which seems out of place, before switching to the title's DuckTales Theme.
This is the original version of the song. It is track 7 in the NSF file and is played on the RP2A03.
The Game Boy release features the same music as the NES, just in slightly lower quality. The song is played on the DMG-CPU B.
DuckTales: Remastered (W32)
This song was rearranged to pure digital music for the remastered game.