My Arrow crab eats Red Bugs...I didn't have too many but he cleared the tank out in a day. Actually he eats anything he can find...slugs, bugs, snails, other crabs
He is about 7" though. I don't know if smaller ones are aggressive as mine.
I bet he would eat the flat worms if they were big enough. Or even small maybe. I have seen him eating pods as well. Best scavenger I have ever had. He keeps the tank spotless.
He eats small fish too.