I'm worried this might be the starting effects of RTN on my acro. I've never experienced it before so I'm not 100% sure. If so, what measures can I take to save the coral? If not, what else could this be? Something maybe picking at it?

My parameters:
SG: 1.025
Cal: 450
Alk: 11 dKH
Mg: 1300
pH: 8.1
Temp: 80

Sorry for the low quality image.

Although it may be STN, my guess is that something nipped at it. What fish and inverts do you have in your tank?

Currently I only have a Midas Blenny and Maroon Clown in the tank. I used to have a Cherub Angel that was a coral nipper, but took him out over a week ago. This just happened yesterday.

As far as inverts are concerned, two sexy shrimp, 10 blue leg hermits and a few scarlet hermits.

I don't know if this helps or not, but I've been doing some reading on RTN and supposively it spreads extremely quickly. I noticed this acro was like this yesterday morning at around 9:00 am. The white hasn't spread from that point. I've heard RTN can spread over the entire coral in less than 24 hours. So, I'm not really sure what to think...

I have no idea what could have been nipping at it either.

What measures should I take?

Although it's rare, midas blennies can nip at SPS.

I think there's a few blennies that look like midas blennies that can be pretty bad at picking on SPS as well...

RTN = rapid tissue necrosis.. a lot of people use this term incorrectly.. In fact, you won't even be able to tell in most cases that you've RTN'd until you go...wow wtf happened here..your coral will be a white skeleton overnight or in some cases even less time.

This looks like a sting from another coral or chomped. Have anything near it, recently moved the frag and bumped it into another coral.. all it takes is just a touch or brush against another coral for even a split second. Shouldn't spread, but if it does, be prepared to hack off that part in attempts to save the rest of it. If it doesn't spread, it'll just die and new growth will encrust over it. Just watch that algae doesn't start to spread on it from that part.

Hmm. I'll keep a close eye on the Midas then. Though I have never seen him be any less than a model citizen.

The closest stinging coral to it is a frogspawn about 5" away. I figured this would be enough distance at their sizes. I have been moving frags around the last couple of days, but I'm usually very careful going about that.

I'll definitely keep an eye on it. I appreciate all the help, I'm relieved that it doesn't seem to be RTN.

Does the flow move from the frogspawn towards the acro? If so then they can reach a good 6"+ sometimes.

Nope. It moves the opposite way.

Though I did find something interesting last night. I set my alarm for around 3am to check and see on the coral and I noticed that the two sexy shrimp were on it. They didn't see to be doing any harm, just walking up and down the acro. Have they ever been known to nip at SPS? And could they have caused the damage shown in the original post?

Bump.

as miketr stated. if it was RTN you would have been WTF happened to my acro by now. Chances are something is nipping... monitor your tank. idk if sexies will do any damage like that though

I dont think that is from nippin looks like maybe a frogspawn falloff hit it, Kalk, or maybe detritus layed on it for a few hours, it doesn't take much, it'll either heal up or die, don't think there is much you can do, WC maybe just for laughs. LOL