You will be journeying close to the top of the mountain for this star, so begin the trek until you finally get to the ivy patch with some Goombas. To get up the mountain a little quicker, head backwards at the beginning and jump off the edge beyond the 1-Up there. You will get boosted by a gust of wind and be deposited near the Chuckya, eliminating a decent chunk of your ascent. Anyway, once you reach the ivy patch with the Goombas, you will spot a familiar purple switch just beyond it. Hitting this makes a breakable block appear next to the switch, which is useful for collecting those coins.
However, what you might not see is that the purple switch also creates another breakable block in the waterfall by the thin ledge just ahead of you. This block will be a good indication that there is a star located in the waterfall. So, to do a quick recap, you will need to hit that purple switch, then quickly run over and jump on the block, which lets you grab the star. Note that it is possible to get to the star without needing that block; you can do a jump kick to gain a little extra distance.