Everyone has already confirmed the sulfur being the bugs you saw, so I will address the indicator nymphing stuff.
Indicators are vital to being succesful on the Clinch consistently along with a perfect dead drift. Plenty of folks catch fish swinging flies and dragging flies, but to really catch good numbers of fish consistently an indicator/dead drift is required.
As Rod stated there is only one indicator IMO and that is Lefty's Yarn, nothing I have tried even comes close to being effective and I wouldn't waste time with any of the other 8 million brands or kinds.
Set your nymph up to ride about 12 inches shallower than the depth of the water, and the most important part of the set up is to have your tippet know located directly below where your indicator is placed. Having your indicator on up the leader in the heavier sections is worthless and will not allow you to get good drifts, good presentations, and will result in poor results.
Historically casting upstream unless you are in riffles is a waste of time on the Clinch no matter if you are fishing dries of nymphs. A good across stream presentation of downstream presentation is by far the best approach. But you have to learn how to dead drift flies by shaking slack line at the same speed as the current or casting and slowly walking the same speed as the current to get the dead drift you need to have consistent success.
On the sulfur fishing stuff, it is a challenging game and as others have stated watch the rise forms. If you see swirls or bulges the fish are generally eating swimming nymphs, and not emergers or adults. If you see a lot of splashy rises or tails they are eating emergers, and if you see noses they are eating adults. Watching the fish for a few seconds can often reveal what pattern they are in, if you watch adults go over a fish with no take, yet the fish is rising in between adults than they are more than likely feeding on emergers or swimming nymphs.
This stuff can get pretty complicated, but over time it will become clearer and don't be afraid to socialize with others on the river as they can generally provide handy tips as to what is going on.