It's a bacterial infection (Vibrio) and it will kill your shrimp, especially if it’s reproductive. When they are young their molt cycle is shorter, thus they molt and get ride of the infection before it does too much damage. You normally see this on their gills or on the exoskeleton, as yours presents. Some species are very resistant to it, others are very susceptible to the infection. Lysmata grabhami and Lysmata amboinensis are very susceptible to the infection. Mortality is temperature related, these guys don't do well with temps over 76 when an infection starts. I personally feel that vibrios like this don’t show there ugly head until the water gets warm. You do see them come in from the wild with this sort of thing.