Dear Blue Lobster:
We have an electric blue crawfish and our frog try'd to eat it! What do we do?????!!??????~~~!@!!!!
Dear Gentle Sir:
This is serious business. Crustaceans and amphibians are natural sworn enemies, each trying to devour the other at any given opportunity since the beginning of their lineages. This started in the Devonion, when pisciforms first began their incursions on land, shallow pool-dwelling crustaceans being an abundant and natural prey. Since then, crustacean and amphibian evolution has been an arms race in which each side develops defenses against the other.
In your case, you likely have an odd pair. Most crays in aquariums are either species native to the Americas or the owner's locale, while most aquarium frogs are from Africa, which lacks native crayfish species; the two species in your tank have not encountered one another in millions of years. In this clash your frog and cray are both agressing against one another in order to establish either dominance or dinner.
For your cray's sake, separate them until they are roughly of equal size. Once neither has a clear advantage over the other, they will keep to themselves. You can achieve this by either using separate tanks or installing a tank divider, a cheap solution available at any pet store.