I explored the idea that they could be idiopathic mucus plugs with the OP, as these are pretty common on purple tangs, and are caused by stress (copper?) but I’ve never seen a case this bad before.OP says spots present for 6-days now. No way Velvet would take that long to kill. It must be something else, although I agree the spots in the photo appear too small for typical Ich.
Any chance this is some type of water quality irritant and not a parasite at all?
People aren’t familiar with late stage Cryptocaryon and often confuse it with Amyloodinium. I’ve seen ich to the point where you couldn’t even see the individual trophonts any longer...
Jay