Acknowledging Co-workers in the bathroom:

It is best to never acknowledge another person's existence in the bathroom. 

If you are going to do it though, a subtle head-nod acknowledgement is next best.  That is the only manly acceptable communication in a restroom.

If you must be an idiot and open your mouth, a one syllable greeting may be acceptable:
"Hey"
"Hi" 
"Sup" 
Not "POOP!"

Using someone's first name is taboo in the restroom: 
"Hey Steve!" is just downright rude, because you have just essentially outed Steve to anyone lurking in the stalls.