Any mutation that is expressed as the phenotype is visible to natural selection. - right - but - by far the largest proportion of mutations AREN'T expressed in the phenotype. They have no effect, they are just uncaused changes in the strands of DNA that make up that type of organism. Remember, most of the DNA in your chromosomes doesn't code for jack that has anything to do with you. You are a mere side-effect, as are blades of grass, antelope, elephants, ants, E. coli, or whatever. Make more DNA - but first, make an elephant.