From the Super Mario Wiki
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A Cloud Block is a semi-solid block mainly used as platforms for bonus areas above the ground. They first appeared in Super Mario Bros. in World 2-1, in the bonus area accessed by hitting the secret beanstalk halfway in the level. Cloud Blocks strongly resemble Lakitu's Cloud.
Super Mario Bros.
Super Mario Bros. 3
In Super Mario Bros. 3, Cloud Blocks first appeared in the sky in World 1-1, reached by flying to the top of the level with a Super Leaf. From this game onwards, Mario can pass through them from the bottom.
Giant Cloud Blocks also appear in World 4.
Super Mario World
Super Mario Maker / Super Mario Maker for Nintendo 3DS
In Super Mario Maker and Super Mario Maker for Nintendo 3DS, Cloud Blocks have a new design for the New Super Mario Bros. U game style. Cloud Blocks can be destroyed by Thwomps, Bowser in Super Mario Bros. 3-styled levels, and Skewers.
Super Mario Run
Super Mario Maker 2