Rinse the adzuki beans (red beans) under cold water.
In a large pot, combine the beans and water. Bring to a boil and let it simmer for about 5 minutes.
Drain the beans, then add fresh water and bring to a boil again. Reduce the heat and simmer until the beans are tender (usually around 1-1.5 hours). You can reduce the time by using an instant pot/crockpot or soaking them in cold water overnight.
Once the beans are tender, mash them or blend them until smooth.
Cook over medium heat, stirring constantly, until the mixture thickens to a paste-like consistency.
In a separate pot, dissolve the agar-agar powder in water (3/4 cup). Bring the agar-agar mixture to a boil and let it simmer.
Stir in sugar and a pinch of salt for another minute.
Add the sweet red bean paste to the agar-agar mixture. Heat the combined mixture over medium heat, stirring continuously until it is well mixed and slightly thickened. This should take about 12 minutes.
Pour the thickened mixture into a square or rectangular mold, smoothing the surface.
Allow the Mizui Yokan to cool to room temperature, then refrigerate for about 2-3 hours or overnight until it sets completely.