Surprisingly, I can't find any studies at all on moray eels and copper toxicity; the only related study I've come across so far dealt with freshwater eels, and found that high copper concentrations didn't impact species abundance in the areas tested (interestingly, they were found more abundantly in areas with high mercury concentrations though).
That said, I know lion king believes/d copper would drastically shorten the lifespan of predators exposed to it - I'd guess that's plausible, but probably unlikely.
Jay Hemdal took some issue with that belief, though, noting that he's only found acute toxicity issues, not long-term issues in most predators (I'm not sure about eels specifically here though).
When I don't know, I defer to the pros and those with more experience - so, to quote Jay Hemdal:
Importantly, though, hyposalinity isn't effective for velvet, which seems to be more important for eels:
So, given the risks associated with velvet, my personal thoughts currently (subject to change upon receiving more information) would be to treat the eel for internal parasites with Prazipro and Metroplex (or General Cure), then run the eel in a 45-60 day observational QT to feed train it, fatten it up, and observe for signs of velvet (or other issues) - the main reason for the lengthy observational period being to ensure that any ich tomonts the eel brought it have died off and that the eel seems healthy before adding it to the display tank. If velvet pops up (hopefully in the QT, not the DT), then I'd probably try treating with copper and hope for the best though.