Starfish are known to get some parasites.. But if you are qt the fish due to something like Ich or Black Spot adding a starfish should be ok.. Another thing to point out Starfish as well as alot of inverts are very sensitive to even small differences in water parameters.. Make sure that if you get one you drip acclimate it as some are even more sensitive than others such as the "Blue Linka"
HTH,
James