I noted the ghost eel going after fish primarily.
This is a kind of critter whose food-response seems to be visually triggered---ie, he sees it, it moves, it WOULD fit within his jaws [he can't bite off or chew] and go down his throat, and therefore his appetite turns on like flipping a switch. This is imho why they're hard to feed, because that food-response is size/motion triggered.
The crab might actually be [counting the legs] a non-trigger, because of overall size. A lot of critters have defensive postures that make them go oversize for the visually-triggered predators in their vicinity. Hard to say. Depends, probably, on getting a larger crab.
The gape of a ribbon eel is huge, relative to head size. But if you get a fish that somehow exceeds it, that fish should be safe.
Your eel is probably quite nicely settled in: the big problem with feeding these fish is simply that you can't get them to eat what doesn't move. But you may have to look at mollies soon.
