Here is an example I have set up to use as a deferred decal that works with translucency, and acts as a masked/translucent decal actor like a rusty stain.
Deferred Decal - Material Set Up
Here you can see I have connected my texture samples alpha channel to the Opacity input and assigned a constant value for the Roughness.
Here is what the decal looks applied to a surface.
Let me know if you have further questions.
Cheers,