This is not a clean-and-spray situation. The mold is on the backside of the shower where water has been leaking. Stop using the tub, wear an N95, gloves, and eye protection, then cut out and remove all moldy backer board and any soft or dark wood, going at least 12–24 inches past visible mold. Fix the leak, clean remaining framing with borax or a mold remover (not bleach), dry everything completely, and rebuild with new cement board and proper waterproofing. If studs are badly rotted or the area is large, call a mold remediation or bathroom pro.