I try to use decals to break up repetition on my concrete material.
At some locations, this “white cross” appears on the decal and I can’t get rid of it.
It’s a leakage decal material from Megascans. I’d be really thankful for any ideas.
I am having the same issue. How did solved this problem?
Increase the roughness intensity 
That’s interesting, the problem might be with material or world light.
In the world Outliner you should have light of your world, so click on it and try change these settings in details and after that rebuild light by default.
If this first method not working you should open blueprint of this material and try change something. Try it out the best learn with unreal is practice.