From the Super Mario Wiki
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Mr. Eraser is an ally in Super Mario Maker, who takes the shape of an eraser with legs and eyes. His icon is placed on the right hand side of the level creation screen, under Coursebot. Upon his icon being tapped, the player is able to remove spawned enemies and objects. He has three official courses made by him.
As seen in the game's E3 2014 trailer, there was originally an eraser that had a letter "E" on it, similar to the one in Mario Paint, which had the exact same effect. This old icon was later replaced with Mr. Eraser.