I agree with Tas trekker, I didn't see an obvious track branching off the one to Stacks Bluff.
The photo shows the approximate location of Coal Falls from the Stack Bluff track near the top of the ridge near Denison Crag. The small lake is Tranquil Tarn, and Coal falls is more or less directly behind it in this view. I suspect that light brown area is in the creek bed.
While I've not been there, it looks like there would be a lot of scree to negotiate. The forested areas tend to have an understorey of mountain berry and other prickly bushes, so if you're off track it might not be the most comfortable walk.
Wello