Is it safe to use Greenboard drywall in a shower?
Green board is basically sheetrock with a waxy paper face. It's not as stiff as cement board, nor as waterproof, both of which are nice to have when applying tile.

CAN you use it? Sure. It's been done before. Is it to code? Not sure. You'd have to check with your local authorities. 

If you decide to use it, see if you can find a product called 'redgard'. It's a paint-on waterproofer that should help keep the green board dry.