With plants and from what you've told us, I wouldn't worry about cycling, that's done with thanks to the plants.
The angelfish is clearly stressed from something. I would look at the other fish first. Sit quietly in front of the aquarium for an hour or so, to get a better picture of fish interaction. Watch for any fish--and from what you have it might be one or more of the other angelfish, the gourami, or the
Red Tailed Shark--causing fear in the lone angelfish. This doesn't have to be physical contact, sometimes if an aggressive fish just approaches the subordinate fish it will be obvious. Let us know if you see anything.
I would think aggression the most likely cause, given your description; if the angel was suffering from some disease it might behave similarly but likely would not be eager to eat. But keep an eye on the tank.
You are wise to be removing the shark, this fish can get very nasty as it matures, it will attain 6 inches and frequently dislike striped fish and any other substrate fish so it is a loner. The gourami is hit and miss, not the best with angelfish but sometimes... again, observe. And definitely lower the temp, 77-78F is fine here. Wild angeelfish need higher temps, but commercial raised fish are better lower.
Byron.
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