My orchid dottyback has a weird white patch on her side....

I noticed it awhile ago...it was a small round white patch. I took a picture and showed the guys at my local fish store, and was told that it looked like she may have been bitten by another fish or got scratched on a rock...but that it didn't look like anything to worry about. Well, at this point it's gotten much bigger....and when I was looking at her yesterday, it looks like it's eating away her side...right where the patch is. She's acting totally normal, but I'm super worried about it. I'm posting pictures of it, however, she is really hard to get a picture of.....it's the best I could do...

Picture #1 is what it looked like when I first noticed it. (a few weeks ago)
Picture #2 is what it looked like yesterday

Any help is greatly appreciated!!

 

Hard to say from the photos, does it look like an ulcer?

 

I’m not sure...I’m new to this whole fish thing...honestly, the best way to describe it is, it looks like a drop of bleach fell on her (which of course is not the case). But, it’s not gross to look at...however, it is concerning because it looks like it’s eroding away.

 

Oh ok, probably not an ulcer then, so that's good. They can lose colour for a number of reasons, although I was always under the impression that they didn't lose colour in a patchy way - more of an all over thing. Have you checked your water parameters? Anything that could have stressed it out recently?