No the yellow stuff in crabs is not poop although it looks like it.
Actually the yellow stuff in the crab is the crab's hepatopancreas.
The crab's hepatopancreas is the crabs organ that is responsible for filtering the impurities from the crabs blood.
So basically the crab's hepatopancreas which is the yellow stuff in the crab is like a humans kidneys where the kidneys filter toxins and impurities from human blood.
The crab has the hepatopancreas which does the filtering of the crabs blood.
So no you're not seeing poop in the crab although it does look like it and the yellow stuff in the crab does look disgusting but it's not poop.