OP:
I've EXACTLY the same problem you had, on the base of a colony of what I believe to be either an A. Cerealis, A.Valida or A.Nana (can someone help me ID it?)
I've had this colony for well over 1 year, from my old nano-reef tank, and it made the move to my new 65G without much problem, except the color faded a little (losing the purple tips after the move), and about 1-2 months ago it start showing the same "STN" in the base of the colony.
When this whole thing happened, the top part of the colony was in very good shape, and didn't seem to be bothered by the STN happening on the bottom, it was showing very good PE, coloration was improving with the purple tips coming back (after I put it under my 15w LED bulbs with optics, measuring some 1000 PAR), and since it's encrusted onto the live rock there's no way I could take it out to dip, so I simply didn't do anything.
At first I thought it was some AEFW, I've used a turkey baster and give the base a few good squirt and didn't see anything get blown off, and I also didn't see any bite mark anywhere else, so I figured it shouldn't be AEFW.
I've then tried to keep my Ca/KH/Mg as stable as possible (I'm on dosing pumps), my Ca is around 400-420, KH around 7, and Mg around 1350. Then two weeks ago I've start seeing new growth along the edges of the STN burn marks, and to my relieve it seems like this STN had stopped.
Here's a recent photo of the colony: