Looks like a Stiphodon by the body shape and fin configuration, these fish have a cylindrical or squarish cross section but normally Stiphodon only spend the small portions of time in salt water, all of the 30 known species live in fresh water and make trips to the ocean (amphidromus). The other problem is there are no species with this color pattern that have been described so I don't think that is the correct genus, but maybe something very closely related.