SPS Help exposed skeleton
 

I bought this piece about 3 weeks ago at a local retailer and it was doing really well. I noticed this the other day and was wondering what it was? Is this STN? I doesnt seem to be getting any worse than 3 days ago. This patch was about .5" - .75" away from a small mille frag and thought maybe it was from the mille stinging it so I pulled this frag and put it on my rack.
http://i53.photobucket.com/albums/g7...n/IMG_0650.jpg

Keep an eye on it, but yes that does look like stn. almost looks like it has stopped though

Have the polyps been coming out at all or does it look like that all of the time?

No I usually see polyps out. I pulled the piece to the front before I snapped that picture.

Was the skeleton that green right away? If so then it has a kind of boring (by "boring" in this case I mean "drilling" and not "uninteresting") algae. Basically if SPS sloughs off tissue (RTN or STN) and the skeleton isn't pure white, then the coral has this and it will have a pretty uphill battle ahead of it. If on the other hand it takes a few days then that's just regular old opportunistic algae and not (in and of itself) something to worry about.

When I noticed it it was green. I have some other skeletons in there that look exactly like that bare patch. I didnt notice this die off until those other skeletons were introduced.

Many times green skeletons can signal phosphate issues, latent or otherwise present in the water column.

This does look like a coral had stung it. But the necrosis has stopped. There is no white edging of the coral tissue, so you are in the clear. As you said, the polyps are out.. which is a very good sign.

I won't worry about the coral. Just give it time, and it will recrust over the exposed skeleton.