How to Complete Under a Red Moon
Travel to the southern side of Washa’s Bluff, south of Tabantha Bridge Stable. You will see a large mushroom-shaped column with a hut, and Kass playing the accordion on top of it.
Speak with Kass and he will point out the pedestal beneath the column while singing a verse about its puzzle. The solution to the Shrine has to deal with the blood moon that happens once in a while.
To force a Blood Moon to happen (or more accurately, in order to wait for one to happen without driving yourself crazy), you can go to Dueling Peaks Stable. One of the NPCs there, Hino, can predict when Blood Moons are about to happen. Ask him about “tonight’s moon”, and if he says anything other than “I’m getting that feeling again…”, sit by the campfire and wait for the next day’s noon. Once Hino says that he’s getting a feeling about a Blood Moon happening tonight, return to where Kass is as quickly as you can. Set up a campfire or just chill out until night falls.
You could also just spam wait by campfires, but this can take forever if you’ve not long had a blood moon. The frequency of blood moons is between 2-3 hours real time but it’s also tied to how many enemies have been slain. So, perhaps a better way to pass the time would be to do some good old fashioned monster hunting!
Once you see the blood moon rise above the horizon, take off all of your armor and stand in your underwear on top of the platform. This will solve the puzzle and the Mijah Rokee Shrine will appear nearby, completing this Shrine Quest.