Decal opacity question

I’m very new to using UE4, and I’m having a problem working out how to make a decal with a material that has opacity aspects to it.

I just want to make a decal of cracks in the ground. I have a crack material, and can place it as a decal, but I cannot remove… well, here is a picture.

I’m sorry I do not understand the lingo just yet, but hopefully by viewing the picture you can see what I’m trying to do. I just wish to remove the texture around the cracks, instead of having the large black square around it.

Thank you.

56370-crack4.png