Sand Sifting Stars

[Snorkel in my tank]

Hey everyone. So my sand (mix of crushed shells and sand) is full of brown algae, green cyno bacteria and detritus. Who recomends adding a sand sifting star? My aquarium store recomended it. Will it compete with my mandrin for food by eating a significnat amount of the little guys he grazes on? Are there any large draw backs to sand sifting stars, or would it turn over and clean my dirty sand well, and becuase of that be a good thing to add? I'm trying to aviod hermit crabs and cucumbers becuase of the damage they are capable of doing. Thanks for the feedback.