Gray Brick Block
From the Super Mario Wiki
A Gray Brick Block is an object appearing in World-e of Super Mario Advance 4: Super Mario Bros. 3, more specifically in the levels A Sky-High Adventure and Puzzling Pipe Maze. It is a solid block that can only be destroyed using the Hammer Suit or the Tanooki Suit's statue mode. Unlike Brick Blocks and Ice Blocks, they do not glisten.