I am assuming the tank is filtered, correct?
Slime coating can look anywhere from translucent white-cottony to close to white fungus patching with some clearer "trails".
You may not need the AQ salt, if you continue to use it, make sure you do daily 100% water changes, and don't use it for longer then 10 days as it can cause health problems such as kidney shut down. (1 tsp per gallon, pre-dissolved)
If it was 3 years ago then I wouldn't say it has anything to do with it now.
He is at the mark for their life span - 3-5 years is the average. So he may not be tolerating as much as he did when he was younger, and he is reacting to something, some form of toxin or possible bacteria that he is trying to protect himself from.
Stay away from Stress Coat at this time, as it will actually increase the amount of slime coating he is creating right now.