Emerald crabs are primarily algae eaters. I have read that if you start allowing them to eat a lot of meaty foods, there is a good chance they will develop a taste for your fish as well.
I feed mine the Hikari sinking algae tabs and he loves them. I break one up into thirds or quarters and give him a piece on long tweezers and he takes it right from me and chows down. When it starts to soften he packs his legs with it and will go hide, relax and enjoy a meal.
The hermits love those tabs also.
He also chows on the chaeto algae like spaghetti.
Were you making sure he had plenty of greens in his diet?