Your red leg hermits are looking for algae to eat, they are picking off what they can find on the coral stalks. I cant tell you with certainty they wont harm the coral but I think it highly unlikely. I've not heard of starfish being harmful to acroporas, although some creatures can display behavior not necessarily associated with its own species. What does this starfish look like or do you know the species? If its as small as you say I think it would be difficult to remove all (unseen) ones with any certainly. The only complaints I've heard was about chocolate chip stars, but I've had them and never had a problem at all with mine. Without knowing where your getting this information form, I cant answer your question.