I definitely think you should consider throwing together a cheap QT. I agree with Bruce, there is a good possibility you are dealing with "suppressed" velvet. The problem with stores using copper is that it's often either a) sub-therapeutic or b) not treated long enough to fully eradicate parasites. This can actually mask symptoms.
I would absolutely suggest a fw dip in route to QT to check for flukes (also very likely with this particular fish). Preferably followed by an acriflavine bath to ward off secondary infections. Read post #2 in the link posted above for velvet for emergency treatment protocol.
Also, this for instructions and a detailed video on how to perform a fw dip and what flukes "look" like:
Freshwater Dip
https://www.reef2reef.com/index.php?threads/Freshwater-Dip.248898/