It is the pathing mesh. It is either there due to the footprint of a building, or because you (or someone else) painted it there.
By the looks of it was painted there. As such you need to paint over it with the removal brush. Go to Layer->Pathing (H), select Painted Pathing. Then select Remove Pathing with Type No Pathing. Paint over the affected area and it will disappear.
If it is a footprint then delete the owning unit.