I've completed the Yoshi Sphinx level multiple times, and I've even used the Door sticker to open the treasure room. But I still haven't found a path leading to the +5HP heart. How can I get it?
When you are on top of the Sphinx, near where the Sticker Comet is, walk north/to the top of the screen, and slide off the top of the Sphinx. If you do it correctly, you should be able to reach the HP-Up Heart with ease.
Images sourced from the IGN guide.