West Bali, about 10 meters deep. Body is roughly 10 cm across. Does not move.
I am convinced this is a nudibranch. The features that indicate this are the rhinophores (antennae-like sensory structures at the "peaks"), the cerata or brachial plume (hard to tell which based on this photo, but these are translucent extensions just visible in the "valley" between the two "peaks", if you zoom in). A look here and here shows there is a huge diversity in morphology, but many nudibranchs have some form of these structures. Also, the shape of margins around the edge of the animal matches that of many nudibranch species. These are mobile animals but it is possible it just was not moving during your observation.