If they were my fish, I think I'd be convinced that it was a dietary issue and not a biological entity. I'm only going on what you've reported. But, you have a bit of an advantage while they are in quarantine --- you have their closer attention. You might stand to accomplish the male accepting more diversity while it is in quarantine than in the open tank again. But, I can see no other cause for further treatment.
You can take algae and mince it up into tiny pieces and press it into mysis, then freeze them together. You can get Julian Sprung algae bits, too. Keep trying different things -- even the same thing by different suppliers/manufacturers. I find that fish will tend to gravitate towards one food or another (especially the older ones) but the aquarist should work hard to prevent this (and not give in to their desires).
