Had Mohs on nose and skin graft/flap, not sure. Is the gray color an indication of failure or infection? 4 days post. (photo)

Moh’s surgery on the face often requires surgical repair and reconstruction to reapproximate the skin edges. Flaps or direct closure techniques are used. It is important that you optimize scar healing during this phase in order to reduce longterm scars. For red scars, we use Vbeam laser, followed by fractional laser for scar topography and contour. We recommend Plato’s Scar Serum twice daily for the immediate postoperative period. Fractional laser is safe for scar laser therapy.
There are treatments for swelling and edema of the scars. If there is residual sutures or edema, we advise you to follow a scar treatment protocol that can minimize the time to recovery. Our office specializes in scar revisions for Moh’s surgery.
Best,
Dr. Karamanoukian
Realself100 Surgeon

A flap is using local tissue with its own blood supply to repair the defect. Although a significant portion of the flap is fine it is difficult to tell whether the end of the flap has maintained its blood supply. Best to see your PS for advice. 

The procedure you had is a type of "skin flap" or "adjacent tissue transfer" (an ATT).  The particular type of flap you had is a "nasolabial transposition flap."  A hole on the nostril (nasal ala) is one of the most difficult locations to fill.  The skin is thick and does not easily stretch.  Simply pulling the hole together can pull the nostril up or pull the tip of the nose over.  There are about 20 different ways you could fill the hole, yet the procedure you doctor performed is also our #1 choice in this circumstance.  Flaps usually yield the best results, but the skin at the tip of the flap has the least blood flowing to it, and can sometimes be starved for oxygen and nutrients needed to survive and heal.  Subsequently the tip is susceptible to delayed healing and occasionally some skin loss.  If the tip of the flap is gray after few days, this could indicate this delayed healing and some impending skin loss.  
Often the loss is limited to the upper layers of the skin, and may manifest as blistering/sloughing ("superficial epidermolysis.")  Other times the loss involves the entire thickness of the flap ("full thickness necrosis").  In either circumstance, the best option is to see your doctor, continue good wound care, be careful not to traumatize your nose, and be patient.  When only the upper layers are lost, the wound may heal relatively quickly.  If the skin loss is deeper, you may develop a scab ("eschar") in that area that may take a few weeks to fall off.  The wound should begin to fill in from underneath and from the sides.  Sometimes a surgical scar revision is needed, but other times is not.  Dermabrasion is a nice adjunctive procedure to consider for blending scars on the nose.
Occasional healing issues with flaps are an unfortunate reality that are not necessarily due to surgeon error.  Skin flaps are an ideal procedure for repairing a hole in this location, but flaps have a reduced blood supply because the skin is partially cut and transferred to a different location.  Over time the remaining blood vessels will expand, become more robust, and allow for good healing, usually leading to an excellent result.

This is a flap, and it appears the tip may be failing. Was there any wound care after the surgery?