This page of the guide to Dragon Ball Z: Kakarot has a walkthrough for the Tourists in Trouble side quest.
Time frame: From Evil Emperor Frieza - Episode 1: Hunt for the Dragon Balls - Episode 1: Hunt for the Dragon Balls
Reward: 7,903 EXP, D Medal, Master Korin Plush
Where to start: Planet Namek, as Gohan.
Talk with the tourists and then go search for Maristone. Destroy the nearby stones to get it.
Maristone may fall out of some rocks. This resource looks like a light purple orb - you shouldn't have problems with spotting it. Go back to the quest givers to complete the mission.