Hi everyone. I use blender to model my low-poly builds. When I get it, insert the mesh part, and rescale it, I get this. The outline on the grass is weird as you can see below.
You can see it on the top, top left, and top right. Please help if you can. Thanks.
Hmm… Odd. Did you have this before the scaling or after? You might have to redo the mesh because there isn’t really much I can say and is the mesh all together or two different parts?. If you can probably provide a bit more detail on what is going on I can probably help. 
Should it be as .obj? Or the other one that I used.
it looks like you extruded it inwards when you where in blender
Looks like a case of inverted normals, something which pops up on the DevForum quite a few times. You can see through them on the opposite end, which would be why we aren’t seeing the edge that’s facing the camera here, and the bottom of the grass facing inward is what makes it look relatively normal otherwise, since from our angle it shows up as regular grass?
Luckily, understanding normals and handling situations like this isn’t very tricky once you know what they are.
Weird render problem with meshes
I would note that after fixing this, you may wish to remove those inner grass polys, unless you plan to use that part of the mesh in a situation where the bottom wouldn’t be covered up by a rock.
It looks like inverted normals.
To fix this, just select all faces in blender and then press Shift + N. Then re export it.
Hope it helped
