I am not a starfish expert by any means but I have read that many "sand sifting" stars can have a negative impact on the benficial critters we need in our tanks and once they have depeleted that food source will move on to whatever they can get their arms around. There are a number of threads and articles online about stars and while some point out the benefits of having these stars as a cleanup crew memeber their are just as many that mention issues of them depleting beneficial fauna and eating fish and inverts.
I would just do some research and decide if it's something you want to keep. Keeping the star well fed will likely help curb any issues as well.
I would just do some research and decide if it's something you want to keep. Keeping the star well fed will likely help curb any issues as well.