An *Occlumens* is a practitioner of *Occlumency*, while *Occlumency* means closing the mind.
Being in the same school with two great Legilimens (Dumbledore and Snape), he was only discovered an impostor after the last round on the 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 might've found it out already, right?