Update: Just got back from my work trip and all the other fish seem to be doing ok. Could this have been brooklynella, not marine velvet? No heavy breathing — everyone else is eating well.
https://www.reddit.com/r/ReefTank/s/TGADVuWlsr
The video I saw *could* have been Amyloodinium (velvet). The time to death lines up. However, when there are that many propagules in the water, other fish will be affected as well, the disease doesn't limit itself to just one fish like that.
Brooklynella mostly infects wild caught clownfish. It can spread to other species (like royal gramma) but it always has the symptom of excess mucus production which I didn't see in the angel in the video. That said, Centropyge angels have ctenoid scales and not a lot of mucus, so *maybe* they would show Brook differently.
What other fish are in the tank and still doing well though?