Bleaching leaves the tissue in tact (yes, they aren't made of solid stone) but it loses all pigmentation so it appears white or very very pale. RTN involves the peeling away of the tissue so nothing is left but the white skeleton. RTN is very obvious-- the tissue will peel off and float around. It typically starts at the bottom and works its way to the top in a matter of a few days or less-- sometimes overnight.
Bleaching is typically caused by temperature, Salinity problems, or light shock and will usually reverse after a while if all other factors are fine (water quality, light, etc.). RTN, on the other hand, is a bacterial infection brought on by injury or some parameter that is way off in the tank, often Alkalinity related. RTN is usually a death sentence within a couple of days (speaking from experience).
Check your Calcium and Alkalinity as Craig Lambert suggested.
