The Alien Tag is one of the minigames in Putt-Putt Goes to the Moon. It is from playing that game that you get a maximum of 5 glowing moon crystals, so the game has to be played at least twice in order for Putt-Putt to be able to purchase the rocket.
Putt-Putt will need to play this mini-game in order to get the glowing moon crystals to buy the rocket ship. This mini-game is to the right of the fork in the road (the opposite way where the player found Rover trying to get the steering wheel). The game can be started by clicking on the crater to the right of Putt-Putt and accepting the challenge from the alien.
The game is done by quickly clicking on every alien that appears out of a crater. The player earns glowing moon crystals based on the number above on the wall. It takes three acts of tagging per point; therefore, fifteen tags will get five glowing moon crystals.