Give it a cold shock.
Place it in your fridge for a night or two(wrapped in plastic to contain the
mold). Bring it out and submerge it in pure water (rain water/melted snow)
overnight, then place it somewhere that stays around 50F (an unheated porch
or cold window). In a few days you should see fruitbodies forming. If
this procedure does not work, and the green mold (or other mold infections)
persist, the mycelium from the Shitake may not have taken in the log and
might be a lost cause due to competing molds gaining a foothold before the
mushroom could. It happens sometimes no matter how clean the innoculation
environment was to begin with.