Combine oats, milk, water, and salt in a small saucepan over high. Bring to a boil; reduce to a simmer, and cook 4 to 5 minutes, stirring occasionally, until oats absorb most of the liquid. Add honey, matcha powder, and vanilla bean paste; stir continuously, 1 to 2 minutes, until smooth and combined. Remove from heat and divide evenly into two bowls. Divide coconut flakes evenly overtop. Add additional toppings, if desired.