Testing for copper would be a good idea. However if copper were present death would not take long, and other inverts would be affected. Although testing to be sure is always the best idea.
Shrimp are sensitive to changes. Bad water change procedures could also be a culprit. For example a significant swing in temp, ph, or salinity from fresh saltwater can not only stress but kill.
If your shrimp is molting it indicates one of two things. Either it is growing or it is stressed. Find out what is causing the stress and eliminate it. Stress can be caused from poor diet, other fish/inverts, living conditions, and parasites and disease. Look a bit closer perhaps
29g Mixed Reef, 150w 14k Phoenix MH w/ Moonlights, Reef Octopus BH1000 HOB Skimmer, Ecotech Vortech MP10es, ReefKeeper Lite (Plus), PH Probe, Temp Probe, ATO, Carbon/GFO Reactor.