From the Super Mario Wiki
Sprout is a member of the Blockafeller species in Yoshi's Crafted World. Unlike the other members, he has a small plant sprout on his head (hence his name) as opposed to a full flower and is smaller than other Blockafellers, suggesting he may be much younger than the other Blockafellers. He is one of the four members of the Blockafeller species to be named.
Sprout appears after beating the final boss in the level The Great King Bowser, where he hosts his own minigame called "Hide-and-Seek." In every level in the game (except for boss levels), he hides somewhere in the level and Yoshi has to throw an egg at him to find him. Once the player finds Sprout in all 43 levels, the player earns the Blockafeller costume.