Is my donor area healing normally 18 days after Hair Transplant? (photo)

That's a large transplant.  It will take a few weeks to heal.  It appears that you may have also experienced some shock loss which is usually temporary.  Be sure to follow up with your doctor.

Hello,The donor area after FUE, whether by hand or ARTAS, takes some time to heal.  Most patients heal atraumatically over a period of weeks. As your hairs begin to grow, the donor area becomes harder to see.  Daily washes with mild soap can help keep the donor area clean. If scabs form, mild soap and water can be helpful. We use PRP in the donor area to facilitate healing as well.  
Best,
Anil Shah

3800 FUE leaves a big scar as you are noticing and will take time to heal. If your are losing chunks of tissue and not hair it may be grafts that did not take.

The donor area after a hair transplant will take several months before returning to normalEvery patient are not healing at the same speed.

At this early stage in the recovery process it appears fine bearing in mind such a huge number of Grafts have been extracted you do have a significant amount of healing to do both in the Donor and Recipient Areas. You may well have suffered with some Post Surgery Shock Loss - I think TIME is the key here, you need to wait for the Donor Area to heal and the existing hair to regrow and allow the new grafts to comes through circa 3 to 4 months post surgery at the earliest. I would advise a follow up with your Surgeon around this time and possibly 6 months post procedure as its such a big surgery.