They are fine to have. I bet you have tons of them. They are probably the most common hitch hiker. They come in on anything you introduce to your tank -- sand, rock, macroalgae, coral frags, etc.
What kind of starfish are they? If they are mini brittle stars (with the long skinny legs), then those are not problematic. If they are asterina stars (usually with 5+ legs, short and stumpy, and are found on the glass and rocks), then those guys have sometimes been known to eat corals, but most of the time are fine as well. Neither of those are "baby" starfish; they are just small species of stars. They won't get any bigger than they are now.