The ABC Mantle is beam-resistant equipment that was manufactured for a short period in the U.C. 0130s.
The heat of the beam causes the material to evaporate, and the gas produced in that process has similar properties to the beam-disturbing membrane, causing the beam to attenuate in that space.
In the photo above, you can see a "mist" with a green phosphorescent glow on the front of the Crossbone Gundam, which is the beam-disturbing membrane. Even this amount is enough to cause interference with the beam from the Brand Marker equipped on the Crossbone Gundam's right arm. This may seem a bit counterproductive, but in the case of this unit, pressing the beam outlet against the enemy will cause the beam to be generated again.

ABC (Anti-Beam Coating) Cloak | gappon's Gunpla works