The fleet captain is the player who created the fleet. Only the fleet captain can launch the fleet! Go to Walkr > Menu > Epic > Fleet members and find the player with a crown next to his/her name. He/she is your fleet captain!

The "captain" you see under your account's name uses the number of planets you've discovered to assign your a level and is not related to the fleet captain. Note: your captain level is the same as the one determining your status on the Ranking page.