silversides are like small frozen fish. You just cut them up in small pieces and throw um in (like sushi to fish ) hehe. I never had frozen brine shrimp so I dont know if they soak them in vitamin enrichments, but if not, I'd get live brine, and then add some "selcon' for them to receive the beneficial proteins to make them something other than "candy" for your fish. I'd let them absorb selcon up to like 8 to 12 hours before adding the brine into the shrimp to ensure it's in their bodies upon feeding.
Also you are right if the snail was already dead, they'll start eating um up. As for the spiderwebs, the only thing I can think of it may be some sort of die-off. A picture could tell otherwise. Good luck to you.