The Overwatch Elite are specialized, higher-ranking Overwatch Soldiers.
Sporting a distinctive white uniform and helmet with a single red
ocular lens, the Elite are the deadliest of the Combine Overwatch
infantry.
Overwatch Elites are equipped with improved body armor, made apparent by
the larger neck guard and the increased protection it offers. Their
helmets are also drastically different from those of regular soldiers,
which possibly improves senses, indicated by the two miniature domes
located where the ears of the soldier would be.
Elites are tougher, achieve better accuracy with their weapons, and
inflict more overall damage than regular soldiers do. They typically
carry Overwatch Standard Issue Pulse Rifles, and are able to use the weapon's secondary fire Energy Ball. They use this advantage without hesitation and with deadly accuracy.
Elites typically fight in squads of their own kind, but will occasionally operate with regular Overwatch Soldiers or, in rare cases, Hunters.
They are generally reserved for more critical combat missions, where
their deadly skills and efficiency are required. They also serve as Wallace Breen's personal guards in the Citadel.

Source: Half-Life Wiki