Hello, Heidi, and thank you for your question!
I did a quick search on my online vet resource and found some posts regarding dogs eating dried starfish, so apparently they like to do this!
What I found is that usually you will just be dealing with upset stomach signs--vomiting, diarrhea, etc. This is not because of a toxin or poison, but because of the rough outer coating of dried starfish which can be very irritating to the gut. If your dog ate 2 legs hopefully this will not be as irritating at this quantity.
What I would like you to do is try and induce vomiting, if the emergency clinic did not already advise you to do so. You may still be able to get your dog to vomit this up, it's worth a try. You can give a few tablespoons of hydrogen peroxide orally and they will usually vomit within 5-15 minutes. This can be repeated once. You may get some of the starfish pieces out this way.
I found that in more unusual cases there can be a neurologic toxin called tetrodotoxin, depending on where the starfish originated. Starfish that came from waters around the US do not usually have this toxin. But it can lead to neurologic signs (brain related). So you would see things like disorientation, seizures, etc. I would not be too concerned about this but did want to mention it.
In any case, if your dog has severe GI upset signs or any neurologic signs that develop, you will want to seek vet care for supportive care (fluids, medications, etc.) I suspect that your dog will do just fine, though.
Good luck and thanks again for your question.