Hitchhikers will come in on all sorts of rock, frags, or algae. Most are harmless - best thing to do is ride it out and research the different types of marine invertebrates. There is a good hitchhiker guide on here. The ones you can’t ID, post and we can try to figure it out. Definitely send pics of what you see so we can confirm. Some slugs can be dangerous. The worm I see is a regular old bristleworm - harmless and you probably have a lot more.
The other plant like thing is likely just another algae that came with it, probably nothing to worry about unless it gets out of control.
I’ve had turbo snails do that a lot, bury. They always come out eventually but do frequently die.