Will the small, dark gray dot on my upper cheek go away from the cannula insertion after Restylane two weeks ago?

This sounds like a small bruise which will resolve. If necessary it could be treated with a combination of  lasers, microneedling/PRP and skin care. Consult your Dr. if it doesn't disappear on its own. Best, Dr. Emer 

Cannulas cannot be inserted without opening the skin, usually with a small scalpel blade and this may lead to a small bruise which is probably what this is and should go away by itself

Usually any small mark at the cannula entry site disappears within a few days.  Sometimes there is a bruise at the cannula entry site.  Sometimes a bruise can leave a stain due to iron deposition in the skin. Without seeing photos it's not possible to say for sure.  If it persists, return to see your injector.  Sometimes laser can be done to make a mark disappear.Regards,Dr. Ort

Facial fillers are injected to improve the contours of the face. They may be injected with a sharp needle or blunt cannula. When using a cannula, the skin is poked with a sharp needle and then the blunt cannula is passed through it multiple times. This may cause a tiny bruise at the entry site. Overtime this will heal and fade. The most important strategy to help it heal: avoid the sun. Just like any new wound or cut on the skin, avoiding the sun will help it heal without pigment issues. If there are any questions regarding your filler injection, I would always encourage my patients to bring it up. Safety comes first.