Sardinium
From Inkipedia, the Splatoon wiki
The Sardinium is a new item in the Splatoon 2 single player mode, used to upgrade weapons.
One Sardinium is hidden in each regular level of Octo Canyon, along with being awarded for defeating each boss. There is also a piece of Sardinium hidden in each hub. Collecting Sardinium already found awards the player 5 Power Eggs instead.