Maybe You Didn't Had Enough Iodine So The Little Guy Would Molt. Or Probably It Was Molting And It Got Bother So It Didn't Finish Molting. Like The Guy Said, Its Missing The Back Legs, But He Would Get Them Back As Time Passes When He Molts. The Shrimp Looks Funny.
This typically happens for 3 reasons:
1 - Something attacked the Crab and picked off the legs.
2 - The Crab is malnourished (uncommon).
3 - Excess Iodine is causing the Crab to molt too often. This is the most common cause.
This does not ever happen because of low Iodine. Crabs do not need Iodine to molt.
There is a Yellow Tail Blue damsel in there. The problem happened right after it molted. He had all his legs, then after the molt, he had some missing!
Could be the damsel, although this one is supposed to be one of the more "peaceful" damsels. Maybe it attacked the shrimp after the molt and I missed it.