With my puffer, I used the raw frozen shrimp from the store, defrosted it and fed it by dangling the food without any parts of my hand underwater with the lights off at first. He would not eat if the light casts a shadow or my finger got in the water initially. He became a pig after about a week of that. Maybe you should get one of those algae clips, and clip the live crab by one of the pinchers (probably will eventually discard his arm) to see if you can get him to feed.
Or buy some freshwater fish for just a meal or two, as I know they are not good for marine fish to eat too much of.
I'd be interested in what WPH uses in the food to make a gel, I'd love to gelatinize (sp?) some food mixes to feed my tank.