Flea collars contain an ingredient called Pyriproxifen which is used to kill off the fleas and the flea growth cycle.
The ingredient Pyriproxifen is not safe for puppies younger than 7 weeks of age.
The flea collar is safe for puppies 7 weeks or older in age though.
So as long as the puppy is 7 weeks or older then it's safe to use the flea collar on the puppy.