It’s really hard to tell with stuff like this as even though you don’t have a natural predator, it doesn’t mean that someone isn’t an opportunistic butt. Best you can do is wait and see if anything happens. If it was eaten, I hate to say it, but the second the other one molts it is most likely game over. Only advice I can say is make sure everyone is fully fed so you don’t have any hunger games going on. As cool as shrimp are, you don’t need them to have a successful tank. Due to my hawkfish I’ve lost 3 and won’t be adding anymore.