Unresolved Acro disease?

My mother colony of bali green slimer developed a patch that looks as if someone scraped the outer layer off. Sorry, not the best description, but the polyps in the section are not extending and it looks very pale. The rest of the coral looks happy, and so does the rest of my tank for that matter. The section affected gets plenty of light and flow. I tried to take some pictures but none really turned out. The spot is spreading rapidly, so help please!

 

I'm not good with SPS but it sounds to me like RTA, Frag that branch/spot off & you should be good I believe. Hope this helped :-/

 

Aw man.... its dead center in the middle of a 5 inch tall frag with atleast 10 branches in the way... I would have to really mangle that thing to do that....

 

If it is RTA you need too if not the whole thing will die. A pic would help me & others determine futher info

 

Total crap picture with the flash on but you get the idea.

 

Is it "peeling"? If not it might need more light. I'm no SPS expert but I try lol I'll let someone else sub in

 

It gets plenty of light and has been doing good for the last month or two. I have another frag of the same coral in the tank and it is doing well too... I have no idea.

 

Heres a Bump for you

 

May be rapid tissue necrosis (RTN). If it is you'll want to frag as suggested. How long has it been like that? I ask because it looks like algae has had time to grow on the dead spots?

 

There is no algae on it. It just started this two days ago....