That is classic STN IMO. Time to frag as much as possible cutting above the infected areas. The positives are it looks like you'll be able to start 3 new mini colonies with a little time. I wouldn't chase numbers or try to find too much direct correlation to a parameter. Corals just do this. My humble opinions.
This can just happen sometimes. Happened to some of mine as well. I would advise you to cut the healthy parts away from where its got stn before it spreads further. Your alk is on the high side so it may have come from a tank with much lower alk and is just now getting stn from the stress. The ones I've gotten would usually do the same thing around the 2-3wk mark if they weren't from a very similar tank then if they made it they would turn around and start growing.