sounds fungal or bacterial.
do you feef him a lot of fleshy foods like squid crab and cocktail shrimp?
in this arrises 2 possible problems.
1) if you are NOT, AND ONLY feeding it ghost shrimp and guppies, you little homie is dying of MALNUTRITION. if he is and its that far along, i am very sorry.
2) you are diversifying its diet and feeding it a wide variety of foods, you might have a problem with two much decaying material in the tank, which leaves your critter very succeptable to fungus, viral and bacterial infections. clean and water change (10% - 15%) twice a week, and fluctuate your salinity up a little every day for a week and look for improvements or new problems
3) it could also be something beyond my knowledge
i feed my eel earl a few different types of crab, shrimp, and invertibrates that i buy from my local grocery. stick a piece in a sharpened stick and let him nab it. its good for the eel in that they need a VARIETY of live and fresh foods in order to maintain proper nutrition, and its real fun for my nerdy ***.