Passer and se passer (reflexive) have different meanings.

Passe-moi le ballon is the imperative of the verb passer.

me passe le ballon could be found in a sentence where passer is not in the imperative and has a subject, for example:

• Mon ami me passe le ballon.
  My friend passes me the ball

where me is the direct object pronoun for the first person singular. And the verb is not reflexive in that case.

Se passer (reflexive) means “to happen” or “to take place”.

• Cette histoire se passe à Paris.
  This story takes place in Paris.

• Que se passe-t-il ?
  What’s happening?