Rapid Tissue Necrosis (RTN) and Slow Tissue Necrosis (STN)
Also known as Shutdown Reaction or Rapid Wasting.
Belonging to a group of diseases referred to as "White syndrome" (WS), that appear as white bands or patches of tissue loss. Aka bleaching. Often effecting members of the genus Acropora. Rapid Tissue Necrosis (RTN) is a fast acting, contagious condition where the coral tissue sloughs from the skeleton. With a tissue loss rate of up to 10cm per hour, RTN result in the death of the colony in less then 24 hours.

STN (Slow Tissue Necrosis) - Coral tissue sloughs off at a much slower rate. Typically less then 1 cm/day.

It is said that different strains of bacteria are the main  causes for  RTN & STN. However, this is  not proven. Bacterias will spread through the water column. So removing the coral into quarantine is highly recommended.

 

Solutions​​​​​​​​​​
Remove diseased colonies and fragment branches at least 25mm ahead of any affected areas.  Dip with RPS ALL OUT and contain coral in quarantine tank.

Sources
http://www.masa.asn.au/masawiki/index.php/Rapid_Tissue_Necrosis