It's most likely the case that your Greninja has moves in its moveset that are compatible egg moves for Corphish, and therefore the moves Greninja had overlapped with the egg moves you wanted to pass down from your Corphish breeder, since Pokemon now inherit moves from both parents.
Dittos have no moves except Transform, meaning all moves your Corphish has are guaranteed to be passed down as long as they're egg moves.
If you want to breed Greninja with Corphish, delete all of the egg moves Corphish can learn from its moveset, and the 4 moves from your Corphish should all pass down.
Source: experience