Yesterday I picked up a tiny shell on the beach, thought it was empty. Carried it in my hand an hour, rinsed it with fresh water and then it was in a plastic bag for like the next 8 hrs. When I got home, I took it out & was looking at it & thought wow, what is that red inside? I touched it with my fingernail & it sort of flinched. Now it’s been in a cup of our saltwater from our tank for 24 hrs with a little flake food. It’s currently almost at the top of the cup & idk what to do with it! I wish I had seen it had something living in it earlier so I could’ve left it on the beach. Any ideas what it is? I don’t want to put it in our tank if it’s dangerous for our fish.