It is usually illeagal to collect sea life in the states, but I won't say I havent heard of a Hawiian sucessfully doing so.
Seems Opihi are limpets and eat algae?
How old is your tank? Do you have alot of microscopic life, and good lighting? I'd be worried about how to feed this guy if your water is not already rich with algae. If you can, get a water sample under a microscope and check it out.
Starfish: Depends on the species. There are sand sifting stars that depend on microfauna growing in your sand. There are also the serpent stars that love tender bits of fresh fish. If you're collecting from a place near your home, you do have the option of putting him back if you can't figure out what he will eat. It would help if you could identify the species.