Soybean hulls. Great for grazing, minimal bio load so you can feed and leave for vacation. It's not very nutritious itself, but it will keep the shrimp happy if you are going to be away for a while, or you just want to give them a treat once in a while. This will grow biofilm if left in the tank for a couple days, which IS nutritious for your shrimp.
50g Bag suits 1-9 tanks
100g Bag suits 10-19 tanks
250g Bag suits 20+ tanks