What treasure am I missing?
Solution 1:
There are two keys that you missed near the entrance of the room. For the first one, at the entrance of the room, climb the vines to your left.
For the second, when you enter, go right and start climbing. Immediately after climbing the first set of vines, you're very close to the key. (Look in the bottom left of the screenshot.)
If you make it to the Bloms, you've gone a bit too far. Go back to the right and, without jumping over the large-ish island you had to double-jump to cross, fall over the side of the platform. You should fall into the key.