Eels do not eat for several reasons, from stress to outgrowing the tank, to changing stages in their life. The first thing to do is not to be overly stressed and concerned and start tossing in a lot of random food or poking it with a feeding stick. If you're eel has been eating well, and you've had him for five years, then he isn't starving anytime soon. Eels can go months without feeding. Last year, my two-year old 15-inch moray went almost four months without eating, and then he started eating everything I put in the tank, four times a week. I did nothing differently, put nothing different in the tank. So do not worry about starvation, you will visibly see weight loss before you need to worry.
Seeing as though you've had him for five years, you've probably tried everything i'm about to say, but here it goes... First, check all water parameters, including temperature, make sure the tank hasn't changed. Introducing the grass and caulerpa may have changed your tank's specs. Second, try feeding him later at night, try feeding him when the tank is dark, as eels prefer night feedings. Try introducing garlic to his food. Also, if kept with other livestock, observe the tank and make sure one of his tankmates didn't decide one day to start picking on the eel.
If you're still trying to feed him daily, stop for a week, it'll be hard, but he'll still be there and he'll still be hungry when you try again. Just don't worry too much yet, he's not going to die of starvation for a while, you'll notice it first. You may want to try a couple different types of food, as others on this board will suggest, but just don't do so on a constant basis and stress the eel out more. If he's been eating the same thing for five years, I doubt his diet suddenly changed, but it's worth a try if another eel-keeper has some words of wisdom on this.
