Mystery Box
From Inkipedia, the Splatoon wiki
Mystery Boxes are items obtained through the catalog from Hotlantis. They are similar to the Shell-Out Machine in that they contain random items, including tickets, ability chunks, Splashtag banners and titles, locker room accessories, and packs of Tableturf Battle cards.
The "decorated" Mystery Boxes are received every 25 levels in the first Catalog of the season and give better drops. After completing that first Catalog, every subsequent one will have "simple" Mystery Boxes every 10 levels, with significantly less interesting items.