Don't worry in advance, it might just take them time to learn that it's edible (especially if they've never eaten it before). Some natural shrimp foods you can try are vegetables (cooked or blanched cauliflower, carrots & broccoli, spinach, kale, green peas, bananas, cucumber) and dried leaves (oak, beech, catappa, walnut, birch, mulberry), or fresh and blanched stinging nettle leaves. You can also buy some more specialized shrimp food. They will also probably pick on leftover fish food, algae and decaying plant matter. It might just take them some time to adjust to a different diet than what they've been used to.
Btw, what kind of shrimp are they?