Hm...that is a strange spot. Though it seems bigger than a grain of salt but I do believe these were smaller fish at the moment, correct? I wouldn't stop the treatment you're doing just yet..but maybe make a new post asking for some more help. I'm not exactly sure why it wouldn't have fallen off yet if it was a true ich spot. I don't know about a chemical dip at all. I don't know much about those. I just know about the ich treatment that I went through myself. Does the spot look like the other spots that had been on them or does it appear different at all?
And if anybody else is following this has any ideas? I'm at a loss.
But if nobody responds you may want to try a new post to deal with this spot but I wouldn't stop the heat/salt treatment quite yet until you are sure what you could possibly be dealing with just in case it could still be an ich spot. Better safe than sorry on that.