Nothing is worse than with the OP creates a thread like this but never finishes it out. It just provides no value to future reefers that use the search function. So here is my attempt to wrap this up and present the results.
1.) I did not change PAR, flow, or location - kept everything the same.
2.) I dosed potassium and increased it by 50 ppm...although the measurement was not severely low on the ICP, I wasn't about to trust just one ICP test... and after doing some research, I felt that there was no harm in raising it by 50 ppm from baseline, just in case it would help. Too many of you said low potassium could cause this to not at least try and raise it.
3.) After pulling the coral and examining it (see post and pics above), I cut off the good remaining pieces (wasn't much left unfortunately), and tossed the rest of the coral with the feather duster and worms in the trash...just in case that was the main culprit for the coral's sudden downturn.
4.) I did a series of water changes (small, 5% max), daily, for 7 days, then stopped all changes and let things ride.
Results:
Original picture when the coral started going south:
(looks faded, no polyp extension, color is muted, almost grey in appearance, tissue thinning and STN'ing)
Here is the coral after saving a few small pieces, not much hope at this time:
(tissue is alive, but still muted, no polyp extension, but this was the healthiest tissue left on the original piece)
Here is the coral today. Growth has returned (can't see the plug anymore), polyp extension has returned, color is making good progress. It's just too bad its a 1/3 of the size of the original frag.
I can't say if the potassium was the issue, the feather dusters, or whether the series of water changes were responsible for the improvement. It's just what I tried and it seems to have worked.
Thank you again to all those who provided insights and shared your experiences.
Once again, this has taught me to never give up on a coral...