Hippopotamuses have adapted to utilize two unique acid secretions in their skin.
The first is called "hipposudoric acid" and the second is called "norhipposudoric acid".
Since these are strong acids, they deter the growth of bacteria on the skin.
In addition, hipposudoric acid is colored bright red while norhipposudoric acid is colored bright orange.
The combination of these two acids provides hippopotamuses with a sunscreen effect because these colors absorb dangerous, cell-destroying ultraviolet light.
Hippopotamus milk is most likely pink due to the combination of these acidic secretions with off-white milk (red + white = pink).