There's a node you can add in your decal called "Decal Lifetime Opacity." You can plug that into your alpha of the lerp. If you're spawning your decals with a blueprint, you can set the decals lifetime and fade duration, which will drive that node in your material.
Here's my setup for a bullet decal. The Decal opacity node controls the opacity, normal and the glow. I use a power node to make the glow fade faster than the opacity