Noticed this as well back in mid July when I was last out there. Guessing with the hot water, and lack of oxygen due to less vegetation with the water so low, maybe caused the fish too much stress producing extra mucus (green/brown slime you saw on them) and eventually killed them off? Did you talk to MDC about it?
I agree there. A lot of meat fisherman up there. Im obviously too young to remember how the lake was back that far ago but in my time I have seen a change. Still catch good bass up there but not like it used to be for sure. Stocking yellow perch would be interesting. I've seen them a lot in Bull Shoals. Provide more forage for the fish