What is this strange line at top of my donor zone after FUE? (Photo)

Your donor area will always look "thinner" proportionate to the number of hairs taken out after a FUE harvesting surgery.  The hairs will NOT grow back from the donor area (where it was taken). Your surgeon may have harvested too many grafts in a linear fashion. If this bothers you, it can be corrected with Scalp MicroPigmentation.

Perhaps the FUE is extracted at the top were done so in a linear fashion thus catching the eye as straight rather than random. You would fill in the area with other FUE to appear more random; or even use micro pigmentation tattooing to camouphlage and create the illusion of randomness.

From the looks of the picture your donor area was over harvested, especially in upper portion.  What you can do to camouflage that is have micropigmentation donor to your entire donor hair to mask the over harvesting look.

It looks like the doctor likely drew on a box on the back of your scalp to mark out the area they were going to harvest from. They took quite a few grafts from the upper portion of that box but never faded it, just stopped right at the line, resulting in a density gradient between the harvested area and the unharvested, which as you have pointed out is very obvious with the hair cut short. You have two great options at remedying this; first you can grow your hair out a bit longer which will help mask the overharvested area, second you could have SMP done on the donor area which in the right hands gives really natural results. Hope this helps! 

Hi, It looks to me like the DR.  over harvested in places and the weird thing is is that it looks like he performed an FUT suture line hair transplant procedure in the middle of your scalp. No worries however, they have whats called smp which is scalp micro pigmentation. It is a temporary tattoo that fades every 2-3 years. If your hair color changes so does the ink. Although it really provides patients with a natural result. Gives the illusion of having hair in an area where you do not. 

From he look of it, the "scar" at the top is the upper limit of the FUE harvesting that was done, and the density from the donor site is noticeably lower the area that wasn't touched.  The donor looks somewhat thin.  Scalp micropigmentation may help improve the look.

This line suggests that the doctor who did the FUE took out many grafts along the upper margin of your donor area and produced what appears to be an over-harvested line