It could be a bacterial bloom, caused by excess nutrients in the water, tank cycling etc.
What are your water parameters? Crabs are more sensitive to Nitrite, Ammonia and Nitrate than fish, so the water needs to be perfect. What are you feeding?
Crabs love bloodworms and shrimp and some veggies but food designed for Corydoras catfish is a good source of food that doesnt cause pollution.
Your tank is overstocked also, crabs are territorial and will fight constantly if the do not have enough 'personal space'. You can often see them crammed into tanks by the dozen in pet shops, this works ok because the cramped condition and large numbers neutralises their teritorial nature. In your tank there will almost certainly be trouble.
Do you have land access in your tank? If there is not enough land areas, they will become flashpoints for trouble. I would not keep more than one crab in a 10g, although a mated pair may be ok most of the time, they will still fight.
Can you post the water parameters? nitirite usually accompanies bacterial blooms and your crabs could be in real danger. The pH also needs to be above 7 for proper shell growth and nitrate no greater than 10.
I would also suggest you buy a bigger tank or bring some crabs back.
You would require at least a four foot tank for the amount of crabs you already have.
Ken
