idk what they are eating I have no macro algae. just live rock, corals, a sea hair, and a cleaner shrimpFrom the description I suspect a parasite outbreak, perhaps Uronema. External parasites can lead to secondary infection from bacteria, fungus, virus, etc.
Since you don’t know, I would let the tank lay fallow for at least a month. I think a UV can help, if properly used. It won’t cure a sick fish, but it will control or prevent an outbreak.
Best practice is to quarantine any new fish, or treat them prophylactically to prevent introducing disease to the main display. These require separate tanks.
You also mention an ‘explosion of copepods’. Is there macro algae in the system? If not, what do they feed on?
Here’s one reference on parasites to look over-
A Hobbyist’s Guide to Identifying Marine Aquarium Parasites
Awareness of the symptoms and treatments for other common parasites can save aquarists from disappointing setbacks, unnecessary expenses and the loss of livestock.reefs.com