Mike covered it,so just a comment.  The Captain is covered in Rule 2, and o 
only a captain is mentioned.  I didn't reread then entire rule book, but I
could find no mention of Alternate captain anyplace in the rules.  As Mike
mentioned,
"If the captain is not available because of injury or an imposed penalty,
another player may be designated to act as captain."
  It doesn't say alternate captain, it just says "another player".  I see
nothing that give 'Alternate Captain' any standing at all.
  In Directive for Officials (which is not part of the rules>, Item 17
says, "Check the player list of each team with the official scorer; note
the names and numbers of the captain and designated alternate."
 
  I see no other mention of alternate.  Rule 2 is emphatic about ONE
captain and that "No goalkeeper shall be entitled to exercise the
prvileges of captain."