Area Room Category Name Difficulty Video
Norfair Bubble Mountain All Bubble Mountain Delayed Walljump Beginner Video
After you start pressing the d-pad away from the wall, there's a window of frames where you can press Jump and get the walljump. In order to get past the ledge to the green door, you need to wait near the end of that window.

This also applies to several other overhangs, such as Plasma Room, and Worst Room in the Game (WRITG).