We have greenboard behind our shower wall. There are signs of mold growth on it.
Can it be cleaned up with a bleach solution and then painted with RedGard?
You are trying to seal the wrong side.
There is a reason greenboard is no longer IRC code approved for tile in wet locations without waterproofing. Its because its NOT waterproof, and neither is grout. You can improve grout's water shedding on vertical surfaces with the better sealers.
The Redguard (2 coats, at right angles) must be between the tile and the greenboard.
Other waterproof membranes would also work, (Kerdi or Nobleflex for sheet, Hydroban for liquid)
You must fix the source of the water intrusion to have a hope of saving this wall from demolition. Mold will stop growing if the moisture is removed.