Zenki....is your eel the one in the first link E. rhodochilus, or the one in the seccond link G. tile? also has yours been eating....usually when they are swimming like you're describing (unless its at night) then they arent comfortable, either not enough hiding spots, or its time for you to go brackish...or perhaps upping the SG a little.
Seaman, my eel eats as much as I feed him. He's a G. Tile that I ordered online, as that's not the common "freshwater moray" (E. rhodochilus) they sell around me.
He comes out of his cave when I open the hood and swims to the top of the tank to take food from me. He also actively hunts the guppies and ghost shrimp in his tank. I'd had him probably a year or so when he started doing that. I've kept him almost four years, he's nearly two feet long now from about 5". He is in light brackish, not freshwater, so are his guppies. I tried to take his SG up prior a couple years back and he stopped eating for nearly six months until I took it back down. Go figure.
In any event, most of the information you find on the web about fw morays is the same no matter what name they have them labled as. Large, brackish - NOT freshwater, keep a tight lid on your tank.
Mine eats tankmates, so if yours doesn't all the more luck to you, I have an 84gal that quite often looks very empty, until he's out and about and that tank doesn't look big enough. I definately think 40 is too small. I started this guy in a 55, and it was about a year before I upgraded.