Combine water and coconut milk in a small saucepan. Bring to a boil.
Add agar-agar, salt, and sugar. Reduce heat to low. Stir constantly until agar-agar, salt, and sugar dissolve, about 3 minutes. Turn off stove.
Gently pour mixture over set Milo layer and allow santan layer to set at room temperature, about 20 minutes.
Transfer to the refrigerator and allow it to chill until ready to be served.
Remove from refrigerator and cut set Milo Santan Agar-agar into desired shapes.