Hmm. I have been reading about this, and from what I have read is peppermint shrimp can rip polyps out, (not sure if you have one), and that high alk (I am talking in the 11's or higher) can cause burns and can take up to 2 weeks to show up. Not sure if any of that is true, but that's what I read.
Obviously, if you just got your corals you can't tell if alk burns are the cause, but maybe you can call your LFS, and see if they know the alk levels, but again, I feel like alk burns wouldn't really affect just that small area of the coral.
My guess would be that something came in on your live rock, (assuming you got ocean rock), and since this is your first coral like this, whatever that was, was able to feed off of it.
Yikes that makes no sense...
I will try to explain it better... You said so far you have only leathers and euphyllias and leathers, so whatever I think ate your coral wasn't able to eat those. Then, when you added the coral, it was able to eat off of it.