The only toxic part of a crab is the lungs, the feathery cones that line the side of the body. It is important to remove them all. Throw them away.
After you do this, scrape out the gooey stuff that is in the center of the body's two equal solid parts. The greenish stuff is the tomalley, or mustard; it's the liver. You can eat it, and many love this part of the crab. I don't.
If you have a female crab and you see bright orange stuff inside, that is tasty: it's roe, or eggs, also called "coral" in shellfish. Coral is very delicious warmed and served on toast, or put into crab cakes, or added to crab soups -- it is the key ingredient in she-crab soup.