It's usual to give Prednisolone
which is a steroid
,in the course of Bell's palsy
along with Anti virals. So it seems like you are having steroid induced acne
,but nothing to worry about it will go away with time as you discontinue the medication after completing its course.
In the mean time you can apply Clinagel gel twice a day on the pimples and I am sure that it will help you a lot.
Thanks very much and all the best.