I was curious and found this....btw its not fat.
Q: What is the yellow stuff inside a cooked crab? Some people call it "mustard." Is it fat?
A: Contrary to popular belief, the "mustard" (or the "green gland" or "tomalley") is not fat, it's actually 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 on both sides of the mid-gut in the main body cavity directly under the top shell 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), dioxin, mercury, and poison-inhalation hazards (PIHs) accumulate in the crab's fatty tissues and concentrate in the hepatopancreas. The contaminants, which are colorless, odorless and tasteless, and can increase one's chance of developing cancer, neurological impairments and miscarriage. Women of child-bearing age and children under the age of 5 are at particular risk.
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.
Eating, selling or harvesting blue crabs from the Newark Bay Complex (Newark Bay, Hackensack River, Passaic River, Arthur Kill and Kill Van Kull, and all tributaries to these waterbodies) is prohibited!
In Maryland, blue crabs were sampled in several 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, it is strongly recommend that you avoid the "mustard".