If you already know how to tell the difference please disregard
There is a major difference in bleaching and RTN/STN. If you look carefully at the acro colony it will be obvious which on it is, regardless of it starting at the base or the tips. this will give you a baseline of what steps should be taken next IME.
Bleaching even after it occurs can be reversed, healed whatever you wanna call it. If your acros still has tissue and you can still see the polyps but it looks white, it is bleaching. this can occur from the top bottom middle etc. Often times the underside of your colonies that don't receive light or good water flow will bleach and die off this may or may not be preventable. Some of it is natural as the coral keeps growing outwards and upwards.
If the entire colony is bleaching you can probably attribute it to stress caused by any of many factors such as large temperature fluctuations, major hcanges or swings in pH, lighting etc. This is why sometimes colonies will bleach shortly after being added to a new aquarium. There might be a significant enough difference in light, temp, pH or something else that will cause it to expel it's zooxanthellae.
A safe bet would be to either move it lower in the tank or decrease your lighting for a period of time to help acclimate the coral to your tank conditions. As well as ensure your tank temps are not fluctuating huge levels in a short period of time and make sure your water chemistry is near NSW levels.
RTN/STN is a whole nother creature
I can't tell you exactly what causes it. Stress, parasites, predation, disease all appear to be viable factors in tissue necrosis of acros. I would think it occurs more often than not in wild colonies that are moved into aquariums than second or third generation fragments. Although I have no proof whatsoever.
Tissue necrosis, is just that, you will see the tissue floating off of the coral as it dies. It can occur quickly (RTN) or slowly (STN). It's painful to watch and will make you feel helpless when it happens. You can try to frag pieces of the acro that are still healthy and hope those sections hold on.
As far as the conditions you describe. Moving from higher to lower light doesn't usually result in bleaching. Maybe some browning out or maybe less intense or maybe even more intense coloration.
you mention your water parameters checked fine, but I'd be interested in what the actual numbers are for Ammonia, nitrites, nitrates, pH, Calcium, alk, SG and temp.