Can you use Haunted Plate Mail as a Spirit ONLY when it hangs around all alone on the battlefield?
The rules for permanents, specifically 110.2, says


  A permanent's owner is the same as the owner of the card that represents it (unless it's a token; see rule 110.5a). A permanent's controller is, by default, the player under whose control it entered the battlefield. Every permanent has a controller.


Only permanents have controllers, and 110.1 says "A permanent is a card or token on the battlefield," so yes, "creatures you control" means "creatures on the battlefield that you control."

In addition, it's worth mentioning that rule 109.2 says


  If a spell or ability uses a description of an object that includes a card type or subtype, but doesn't include the word "card," "spell," "source," or "scheme," it means a permanent of that card type or subtype on the battlefield.


Since Haunted Plate Mail just says "creatures," it is referring to creature permanents on the battlefield.