i dont know the time table of this reaction... but i am guessing it isnt RTN... RTN will consume a coral in a matter of hours. if its happening longer than that... say over nite... or over a coupla days... .its what most people call STN. its the same thing, just a different amount of time. there is also shutdown reaction where the coral loses tissue across the whole coral at once instead of just an area here and there.
when you frag a necrosing coral... "over-frag" it... meaning even cut away a little into the healthy area. if you just cut away the dead part the necroses can still spread.
reason i said frog and flow above... hard to tell depth perception in pics, but it looks like the frogspawn could be reaching out and touching the acro.
reason i say flow... when corals start shutting down slowly from the base up, the coral usually isnt getting as much flow as it needs.... due to growth, new corals place near it, rock deflecting water, etc.