Rolling Canyon Seal

Walkthrough: After breaking the third seal, drop off the southern side of Hot Springs Island and begin making your way through the number of tide pools as you battle the kelp creatures that attack. You should also be able to find another Power Moon as you travel, one found simply by ground-pounding one of the circles of water in one of the tide pools. After reaching the shore, you will come to a large canyon with a number of spiky shells rolling down said slope. You can find more Purple Coins by some of the locals, as well as near the Goombas to the right of the canyon.

Head into the canyon, dodging the spiked shell as you do. If you're willing to take a risk, you can about a Power Moon from Goombette within the canyon by stacking together a group of Goombas and heading towards her. Otherwise, continue dodging the shells on your way up the canyon. There will be an alcove on your way up that holds more Purple Coins, but besides that, continuing head up until you reach the top. There will be a stack of crates at the top of canyon, the same you must break through to reach the other side. Out on the cliff will be the final of the Power Moons that will break the remaining seal to open the fountain. After obtaining it, the monster, known as Mollusque-Lanceur, will become enraged.