I played Green Grooves Waterfalls too, it's great! Everything is well placed and the level is fun to play because it has a good flow. There is only a very small "glitch" at the end of the main room, where the note blocks are over the jumping plants; if you jump in a way, you can reach the top of the ground tile at the top-right corner and walk offscreen.
The only part that gave me problems level design wise is the last dragon coin, because those jumping plants will land on the moving platform and it feels cheap... but it's a very minor issue that should be easy to correct (I think).
The difficulty level is balanced, and it's hard but not too much, something that you may expect from a world 6 or 7 of an official Mario game; the difficulty raises a bit if you try to collect all the dragon coins but it's still very doable (maybe, it may bring some rage to the player to start everything from the beginning each time you do a paratroopa jump wrong but that's not your fault, usually in Mario games, if you don't kill a paratroopa, he will fly again when you refresh it by going offscreen or changing room, but in this engine it won't fly anymore).
What I liked very much is the level's lenght, that's just correct, not too long, not too short.
One of the best levels I played in SMBX so far (if not "the").