You can use an alpha mask to make the decal whatever shape you want.
However, the decal will apply to everything, not just the hexagonal base. You would have to disable decals for things above it which means you can’t use effects like blood splatters or burns for it anymore. (or make a very thin decal that slips between the base and the object above it, but it’s prone to artifacting)
I think a better approach is to make the the hexagonical graphics you want to apply a part of your material. You can toggle and blend layers/attributes through blueprint or c++.