By pure luck I stumbled upon this in PlayerCharacter.cpp.
In Constructor there are few lines of code wich I had to comment/delete, to disable cursor decal, here they are:
// Create a decal in the world to show the cursor's location
CursorToWorld = CreateDefaultSubobject<UDecalComponent>("CursorToWorld");
CursorToWorld->SetupAttachment(RootComponent);
static ConstructorHelpers::FObjectFinder<UMaterial> DecalMaterialAsset(TEXT("Material'/Game/TopDownCPP/Blueprints/M_Cursor_Decal.M_Cursor_Decal'"));
if (DecalMaterialAsset.Succeeded())
{
CursorToWorld->SetDecalMaterial(DecalMaterialAsset.Object);
}
CursorToWorld->DecalSize = FVector(16.0f, 32.0f, 32.0f);
CursorToWorld->SetRelativeRotation(FRotator(90.0f, 0.0f, 0.0f).Quaternion());