If you are having ammonia spikes, your tank is probably not cycled. You shouldn’t add any more livestock to your tank until it is fully cycled; in fact, I wouldn’t recommend adding shrimp to any tank younger than 3 months.
The ammonia probably was the culprit for their deaths, but if the algae wafer had copper in it, that also could have been the culprit. Invertebrates are very sensitive to copper and some fish foods contain enough copper to kill inverts. So cycling your tank, and checking your food for copper would be good ideas.
Not picking on you, but much of that is incorrect.
An ammonia spike does not mean you are not cycled - it could mean you don't have enough beneficial bacteria to process the amount of ammonia present. In a shrimp tank, the bioloaod is so low that the amount of bacteria is small. Death of a large snail could produce far more ammonia than the bacteria normally handles. The rate of growth to accommodate ammonia can be slow depending on pH and temperature.
I agree that new livestock should not be added until things are stable again, but the recommendation of only adding shrimp older than 3 months is bad. Younger shrimp actually acclimate better, and we usually shoot for 5-6 weeks old for better survivability.
As to copper, it is only toxic in excess. Invertebrates NEED copper in small amounts in the same way we need iron. If the algae wafer was soaked in a copper based medicine, that would be excess. Most of these foods with copper are safe.
My bet is that the white stuff in the gravel is leftover snail... You might want to use an airline tube or similar as a gravel vac. Your goal would be to vac all water for you water changes to keep the gravel clean. We tend to avoid gravel in shrimp tanks because the only way to really keep them clean well is a UGF which usually isn't good for shrimplets