There is absolutely no empirical backing for the notion - chiefly disseminated by marketers of supplemental products (and reinforced through casual association on the part of aquarists) - that aqueous "iodide" (rather, certain "iodine" compounds) is even bio-available to freshwater shrimp. In truth, even if it were, shrimp requirements would likely be, based on suggested values for comparable aquacultured species, truly miniscule (and therby likely satisfied through consumption of foodstuffs). [Shrimp do require iodine to grow, but only in terms of a dietary requirement. Penaeus chinensis, for instance, grows optimally when its diet contains 0.003% iodine.]
Increased risk of predation is largely responsible for post-molt (while the shrimp is still reconstituting chitin) mortality; however, calcium deprivation may also be a factor.
What are your water parameters? Ghost shrimp often arrive in store tanks highly stressed and may therefore succumb for seemingly no reason (in truth, for what would otherwise be inconsequential environmental factors).