Jelly

From the Super Mario Wiki

Jelly is a substance that makes its first appearance in Yoshi's Story, specifically in 2-3 Jelly Pipe. Here, it can be "dug" into by the Baby Yoshi using a forceful move such as the Ground Pound. Sometimes fruit is trapped in the jelly, also.

Jelly also appears in the World 7 Ghost House in New Super Mario Bros. Here, it can be dug into with a Ground Pound once again in one room. In another room, it comes in "waves" towards Mario in a room filled with Splunkins.