Hi Heroic. Since the slime part has gone away, we can assume it was probably as a result of his fin rot treatment. The graying and the lethargy are a little more troublesome and could be indicative of an internal problem. I'm a little hesitant to suggest a medication since I'm not sure what the nature of that problem is, though. However, you may want to put Red in 1 tsp of epsom salts and if you have Indian Almond leaf or dried, clean oak leaves, you can put those in as well for a source of tannins.
If you did want to try a medication, I would suggest API General Cure or metronidazole as it will treat both internal and external parasites and also provide limited treatment to any internal anaerobic bacteria as well. It's not an antibiotic so we don't run as big a risk of building up a resistance.