Contrary to popular belief, the mustard (or the green gland or tomalley) is not fat, rather it's the crab's hepatopancreas, a main component of the crab's digestive system.

The hepatopancreas is a gland made up of very small branched tubes (ramified tubules) located either side of the mid-gut in the main body cavity (cephalothorax) directly under the top shell (carapace) and functions as both liver and pancreas. It is involved in producing digestive enzymes and is responsible for filtering impurities from the crab's blood.

The mustard has a strong taste and is eaten by many people who consider it a delicacy.

Caution: Research shows that chemical contaminants such as polychlorinated biphenyls (PCBs) concentrate in the blue crab's hepatopancreas. Crabs caught in advisory areas may contain high levels of these contaminants. If you catch crabs in these areas, it is highly recommended that you eat no more than six blue crabs per week and do not consume the mustard or cooking liquid.

As a side note, crabs were sampled in several Maryland areas back in 1994-95, including the Chester, Bush and Gunpowder Rivers, Baltimore Harbor, Colgate Creek, Ft. Armistead, Old Road Bay, and Curtis Bay, and the Patapsco River. With the exception of the Middle and Northwest Branches of the Patapsco River, blue crabs were found to be generally low in contaminants. However, generally low is not the same as zero. When it comes to your health, I recommend that you avoid the mustard regardless of location.