I have three other possibilities for you from my eel bible ( Keeping Moray Eels in Aquariums, by Phil Purser ).
Listlessness and loss of appetite are listed as a sign of bacterial or fungal infection.
Refusal to eat and side swimming are signs of Nitrogeonous waste poisoning as mentioned in previos posts.
Refusal to feed and inability to swim upright, although listed with lumps under skin, would be signs of a tumor. You haven't mentioned anything about any lumps on your eel, so I would hope this is not the case.