Yep... they just love mussels and cockles. And if you're brave enough it's always fun to hold a mussel between your thumb and forefinger and wave it in front of his cave (I take it he has a cave? He SHOULD have a cave, anyway). Just be sure to let go when you see him stretch out his neck
If your G. tile is refusing food try a specific gravity of about 1.005 to 1.015. Their apetites increase with a higher SG as opposed to FW or light brackish water.