There is not an in-game canonical explanation that concerns the origin of the murals, but the player can reasonably assume that the residents of the nearby Triton Village may have had a hand in their creation, back in the ages when their civilization was thriving. The murals may have continued to cover the inner walls of the cave beneath the current waterline, but they have been washed away by the ocean currents at this point.
The Red Dome is a major part of two post-game quests, where Hayako asks the player to help her research the murals further. During these quests, a small group of Great White Sharks appears in this Dome, but these are not permanent residents and disappear immediately after their contextual part in the mission has been played.
Local Sea Life
The two species that make up the majority of the inhabitants in the Dome are Sea Goldies and Japanese Angelsharks, both of which have a colouration on the red-orange end of the colour spectrum. However, neither of them are explicitly 'red', but are instead only colours that are close to red.
"This dome-like structure, found to the west of Ciceros Strait, resembles nothing more than a hollow boulder.
Schools of colorful sea goldies and Japanese angel sharks can be seen here. The ancient murals on the walls provide the interior with a mystical atmosphere."