Green Star Coin
From the Super Mario Wiki
Green Star Coins are Skill Items in Puzzle & Dragons: Super Mario Bros. Edition. They raise the Skill levels of all forms of Luigi, Spinies, and all types/forms of Bullet Bills, Wigglers, Chain Chomps and Flame Chomps from Level 1-4. To increase their Skill Level from Level 4 onwards, Grand Green Star Coins need to be used. Green Star Coins can be obtained by winning the Lucky ? Blocks or by opening a ? Block in certain levels.