Whenever I train for frozen, I usually just use a feeding stick and wiggle it close to them, they may be hesitant at first but they eventually learn the scent and later on I can just drop the food on the top and they'll come out and swim for it.
Stomatopods are opportunist, if they're hungry they'll go for easy food, dead or alive. How much you feeding him? He could have no interest in eating whats handed to him if you're feeding him the snails frequently (he acknowledges snails are always available and expects them instead of foreign content.)
Hard to tell what species that is, looks like a male G. glabrous. Possibly a male G. Ternatensis.