Actually, I don't think you need to worry much about life dying off. Just feed the rock. Pop in some food now and then for things to eat, keep an acceptable light over it for the algae, and so on.
That said, if you haven't heard any very loud clicking (could mean a mantis shrimp), or seen any big crabs, it's probably fine to add. Ich being on the rock is pretty unlikely.
If you keep hearing clicks, I would keep running bottle traps for a bit to try and find out what made that noise. It's probably a pistol shrimp, but confirming it would be good.
Sponges are generally harmless. Some species are fast-growing and can be an issue from that, but it won't hurt to leave them in briefly and see what they do. If they start growing fast, yank 'em.
That said, if you haven't heard any very loud clicking (could mean a mantis shrimp), or seen any big crabs, it's probably fine to add. Ich being on the rock is pretty unlikely.
If you keep hearing clicks, I would keep running bottle traps for a bit to try and find out what made that noise. It's probably a pistol shrimp, but confirming it would be good.
Sponges are generally harmless. Some species are fast-growing and can be an issue from that, but it won't hurt to leave them in briefly and see what they do. If they start growing fast, yank 'em.