Very mild and preventable resulting in facial erosion known as HLLE which cause pits and flesh missing mainly in tangs, angels and some rabbitfish. It is often caused by poor water quality, high use of carbon, poor water quality (elevated nitrate and ammonia) and inadequate/poor diet. It is not life threating in any way but offers secondary infection in some cases.
Maintaining GOOD water quality and diet are often the fixes and with severe cases, some healing.
Some foods to feed tang are :
LRS herbivore diet
Formula 2 flake and frozen
TDO Pellets
small plankton
Hikari Marine cuisine
Ocean nutrition veggie diet
spirulina brine shrimp
mysis shrimp
Prime reef
Nori seaweed basted with garlic extract
Add selcon vitamins to foods occasionally