I agree it is velvet.
I had a very odd strain of velvet that showed NO outward symptoms (other than a little ich off and on) on fish that are obvious. Fish, particularly angels and tangs, would die within 2-4 days. The water params were flawless. This persisted each time I was sure I found the culprit (I did have flukes and I treated for them). The losses continued. I hadn't bought any tangs until my Chevron, and unfortunately he had to take one for the team. I was already wondering what the disease was or if it was the water or something i could not identify - and then within three days of his purchase he died, but on day 2 I noticed CLASSIC velvet (even the velvety coating that you see only in severe cases) and everything began to make sense.
PERFECTLY healthy angelfish from Divers Den lasted 2 - 7 days. They NEVER showed ich (the tangs did a few spots), NEVER showed any other obvious symptoms other than heavy breathing which occurs in nearly all parasitic infections and other water issues. Why is this relevant? Well my Emperor Angels (both that I killed

) showed those spots.
I hypothesize that velvet affected the gills primarily, which is why there were no obvious external signs until the day of death when a milky film covered them (I thought it was just color loss because they were dead or nearly dead - I had not seen velvet in YEARS). But because the parasite so heavily infests the gills, the fish is not able to get proper oxygenation and coupled with stress can cause those splotches.
Within ten minutes of identifying velvet on the tang, I treated the tank with Cupramine. I am continuing treatment, and have ordered some replacement fish I will keep in quarantine with copper as well until I am certain the tank is cleared up. Velvet is horrible, i had i about 8 years ago and that stuff is like ebola - it cleared out 4 tangs, 2 wrasse, 2 chromis, 1 trigger, a mandarin, two angels and I cannot remember what else in less than a week. My Melanarus wrasse showed NO symptoms whatsoever, and one of my chromis did not either. They lived several years after until I sold them both.
So confusing also was the fact that my Banana Wrasse, Red Coris Wrasse, Lime Green Wrasse, Cleaner Wrasse, Candy Cane Squirrelfish, Lawn Mower Blenny, and Sleeper Goby showed no symptoms at all. In fairness, the sleeper goby was only exposed to velvet for 3 days before I started treatment, but seemed completely at ease and normal. But this makes sense, wrasse (except leopard, fairy, and flasher wrasses in my experience) are often able to live with velvet with little or no impact.
My Magnificent Foxface got cloudy eyes but recovered with velvet in the tank and started acting completely normal, so she too seemed to build an immunity. Poor fish were probably still suffering, just not obviously. She is still kicking and thriving.
Good luck, sorry for the essay but velvet is getting sneakier, and it is VERY deadly