Dry wall in shower a little moldy
Drywall shouldn't be in the shower.  Take it down and replace with cement board.  This will cost you $50 and will ensure the money and time you put into the tile last more than a year.

If for some odd reason you are using a Kerdi membrane or something like that (which your question leads me to think you are not) than you could keep the drywall.  You would have to scrape off the mold and hit the drywall with a anti-mold spray.  If the mold doesn't scrape off then you need to replace that section of drywall.