I second the ID of chiton on the first photo. Most of the species are found in the intertidal zones on or around rocks. So most likely it was a hitchhiker on the rocks you added. They eat algae, bryozoans, diatoms and some even bacteria. The larger ones can feed on small invertebrates (Shrimp) small fish etc. I’d keep him if he’s somewhat small. He very cool looking.
The small star fish - I have lots of these and in my tank are harmless- I really like them and watch them all the time. I have the ID on it and will look it up and get back.
BLW - Great photos!