Pickled Mouse Feet

Your new alien neighbours (you keep forgetting how to say the name of their planet) bring over a jar of their own special, favourite jellied dessert topping. It looks like pickled mouse feet but you don't want to say anything... well, not really. Is there a polite way to decline their offering without causing bad feelings between neighbours? Keep in mind they have a pet 'dog' that looks like it could swallow your house, whole.