I also use dried krill and my mantis has loved it since day one. He watches me pick a small piece out and perches on top of a rock until I throw it in the water.
Most of the time he waits until it becomes saturated, swims out to snag it and then returns to his burrow to eat. Other times he gets anxious and pulls it off the surface like a fish. He's very clumsy though when he doesn't have a grip on something. One time he swam to the top, hung on to a snail at the water line, and then went after the food as it floated by on the surface.
Hilarious...