You mean a dechlorinator, right? Like Stress Coat?
It should be fine. The crabs don't need the slime part, considering they'll be out of the water a lot of the time, but they need dechlorinated water.
I got some shrimp pellets, they should work better.Something like hikari hermit crab food or some sort of shrimp food would be better, but the flakes should be OK.
Although, you may know what they need, but I've got to ask, what sort of set-up do you have the crabs in?