When you treat for ick with salt, Nox ick or any other med, you are not killing the spots that are already on the fish.
Those spots are protected by the fishes slime coat and immune to meds.
But every couple days those spots explode into hundreds of new, young ick.
The med you put in is there to kill those new ick that would reinfect the fish en masse.
The fish in the photo has had ick longer than a few days, but when there are only a few spots, they often go un-noticed, some are unseen inside the gill plate.
This is why any treatment can take weeks, because you need to kill each emerging generation so they cannot reinfect, and often each spots morphs into young ick at different intervals..
Another problem is those spots, where the exploding ick was, are lesions that become place for secondary bacteria to take hold, and those bacteria in combination with the ick may eventually kill the fish.
This is why it is so important to catch it early, and not go light on the salt or mess, and not to stop treatment early.
I add 3.5 lbs of salt for every 100 gallons, and that 3.5ppt salinity kills all emerging ick. If you less less salt than produces 3ppt salinity, the ich hatch away and reinfect at will.
And any salt will do, as long as it is NaCl.