from my experience stn/rtn normally goes in a somewhat band/ring-like formation, usually from the base up the coral.. and generally the skin can be seen literally peeling and fluttering on the stn/rtn edge of the coral (versus no skin at all.. higher chance that something's eating it..)..
can't tell much from the pics, but seems like the bad patch isn't going around like a ring around the coral base and there is still some okay live of the coral all around it, including where it is encrusting to the base.. which to me generally means not stn/rtn (though if its rtn/stn'ing i feel like it goes faster at night .. so tomorrow may be a different story..

). also, generally stn/rtn show bone/all white on tn parts.
could be something munching on it, or something touching/irritating (though doesn't look like anything coral-wise would, i'd watch out with the gumdrop goby in your sig though for sps in general, as they like to 'nest' in sps and seem to stress/kill sps tissue locally around their favorite perches)..
how long ago did you get your frag? maybe to be safe, do a iodine/tmpcc dip for a bit and see if anything falls off, then place back in.. if still tn'ing then try to frag off a little more above the deadline (better chance of the tn not jumping onto the rest of the frag.. as seems to move underneath a bit faster than is evident on the surface)..