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 series
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 Run