Umami Crystals are purple items that you will encounter by defeating strong enemies or breaking chunks in the overworld. You can use these Crystals to unlock new skills for Kril, like the ability to fast travel between the different unlocked Moon Snail Shells. These act as a safe haven in Another Crab's Treasure, allowing Kril the chance to rest up and heal, check out the shell collection, level up their stats with Microplastics, and, eventually, fast travel.