How did you acclimate them? I assume these are neos. If the water parameters from LFS/breeder is far off from yours, that could cause failed moltings and even death, even if you have "ideal" parameters. This is usually only a problem within the first day of acclimating them though, even the first few hours where osmotic changes have occured quickly. The white crack is usually indicative of a failed molt. Your parameters seem pretty good to me but again, if they were previously thriving in a GH of 12, then that could be an issue. Also, what is your water temperature? Shrimps generally prefer cooler waters.
As for the shrimp that's acting weird, it could be bacterial or parasitic, so quarantining that one was a good idea. Sometimes adult shrimp have a hard time acclimating to a new tank and they just die for no apparent reason. It's common to lose one or two shrimp at first. If you can get through the first few weeks with them, they'll usually do just fine. Once they start to breed, the next generation is usually quite a bit hardier and your colony will grow.