You need to find him, if he has died you need to remove him immediatly, otherwise his decaying corpse can cause a spike in your tank. It sounds to me like something bullied him to death. A nip here and there does not seem bad, but if you add em up and add stress into the equation it all makes sense. Stress does all kinds of things to fish, such as breaking down their immune systems and not letting the react properly to infections and disease (HLLD is a good example of this). Exposure to abnormal amounts of stress can simply cause a fish to die, they cannot react properly to the situation.
And you absolutly need to get a mag test kit, expecially if your keeping corals, its essential.