My assassins seem to eat whatever frozen food that hits the bottom of the tank...beefheart, bloodworms, brine shrimp, etc. They crawl around all over the place, glass, plants, found two baking on the heater, but they also spend plenty of time burrowed in the substrate.
I have no experience with shrimp, but have read some folks here saying their assassins have eaten their shrimps. Hard to believe, but that's what's been said.
65g planted tank - 200W heat - PC 192W/6700K - Eheim 2217 - Pressurized CO2 - EI Dosing