Hi JimmYaz,
Thanks for the response to my thread. I think I can share a response to your situation as well. But take my advice with a grain of salt. I've gotten to the free-swimmer stage several times, but was only recently able to keep them from dying off.
The easy quetion to answer is the yolk sack. That would not be on their head...at least not in a healthy fry. Also, in my limited experience they can only feed off that for a few days.
If you want to raise the one fry you should get him some food, since he can't feed off the parents now. I've been using egg-yolk mix on my 1st successful batch. Smeared around the edge of the bowl they float in. You could also try Baby Brine Shrimp. I've read that the San Francisco Bay variety is small enough for fry to eat at this stage.
If a fry is healthy it should be free swimming within a few days from wriggling. I'll tell you though some of mine would lay on the bottom for a while, in the first few days. I had to watch at length to see if they could actually swim around.
I hate to ask this, but are you sure it hasn't already passed? It may have died and grown fungus that stuck it to the wall. Hopefully that isn't te case!
Wish you the best of luck. Either way I'm sure you'll get it sooner or later.
Evan