Facial Fat Graft Feels Hard Post-procedure

FAT GRAFTING aka LIPOINJECTION aka AUTOLOGOUS FAT INJECTION
The massage should be guided by your surgeon as opionions vary.
You did not specify the timing of your procedure as this can be important in the evolution of the post operative findings.
Firmness as Dr. Law documented can vary as a result of :

area injected i.e. cheek bone/ tear trough fat will feel firmer than cheek hollows
volume injected
amount of associated bleeding bruising
degree of "fat take" survival

Firmness is common with any procedure for the first 4 days. As the blood supply reaches or doesn't reach the fat cells some will die "fat necrosis" and trigger a process of inflammation and fibrosis (scarring) which will typically develop over a 3 week period. By 6 weeks, this process is generally completed and the areas continue to soften.
Therefore the 3 week period is a good time to assess the results of the lipoinjection procedure. While it may remain slightly overcorrected at this point, a soft area indicates good fat survival whereas a firm area indicates poor fat survival. This may be the result of a variety of factors and not always directly attributable to the technique. For example, a history of smoking contributes to poor fat survival as does previous scarring or surgery in the area. Aspirin or NSAID use may cause greater amounts of bleeding which can also be pericieved as hard
If other procedure have been simultaneously performed in the same area, this can also contribute to sensations of irregularity and firmness.
Rarely the fat may die and form a cyst or an abscess which needs to be drained or aspirated.
I hope this helps.

No, this is very normal. I tend to think when the areas are hard, the fat tends to last longer. Maybe the fat was a better quality to begin with.

  An important missing information with your question is the time since your procedure.  But certainly, massaging the area might be very beneficial and I don not see a downside to it.