From Halopedia, the Halo wiki
Camouflage cloaks are devices employed by the UNSC as a medium of concealing its soldiers in certain environments.
Camouflage cloaks take the form of simple cloaks that can be worn over most armor and equipment, even including MJOLNIR Powered Assault Armor. When worn, the cloak serves to camouflage the wearer's thermal signature from sensing devices.
Camouflage cloaks were employed by SPARTAN-II Blue Team during Operation: OCEAN BREAKER.