One of my peppermints has that on its side too. It has molted at least once if not twice and has been in the tank for about 2 months.
Not only that, the pair have actually laid eggs twice. The eggs hatched both times and the babies were sucked into the filter as well as eaten by the other inhabitants.
The bubble does not appear to harm the shrimp at this point, but it is considerably smaller than its mate (the one that holds the eggs) I dont know if this is a difference of sexes or what.