I had a harlequin and they do not have any interest in anything but starfish. They will devour the tank of them in a month or two. Then you will need to buy chocolate chip starfish weekly as a food source. These guys eat coral so buy one and keep in the sump. Then every 1-2 days cut off an arm and feed the shrimp. Once the star has half its arms gone, give the remains to the shrimp as a big meal. Then, go back to the LFS and buy another starfish
Harlequin can not go very long without eating, you will always need live starfish on hand