An Occlumens is a practitioner of Occlumency, while Occlumency means closing the mind. Despite being in the same school with two great Legilimens (Dumbledore and Snape), he was only discovered to be an impostor after the last round of the Triwizard Tournament in Goblet of Fire:

“Moody,” Harry said. He was still in a state of complete disbelief. “How can it have been Moody?”

“This is not Alastor Moody,” said Dumbledore quietly. “You have never known Alastor Moody. The real Moody would not have removed you from my sight after what happened tonight. The moment he took you, I knew — and I followed.”

Does this mean that Barty Crouch Jr. is an Occlumens? Because if not, then Snape or Dumbledore would have found it out earlier, right?