it is very likely the mold-producing-moisture is coming from behind the caulk. It may be that even silicone won't stop the mold.
Moisture migrates into the grout and sometimes behind the tile everywhere, gravity then takes it towards the floor inside the grout and deposits it [behind] the caulk where it is slow to evaporate. This is typical of how a tile shower lives. The trick then is to keep the area as dry as possible by squeegeeing the walls after a shower and using a ventilation fan to evacuate the moisture in the room and shower.
In some cases it just happens and without sufficient air-handling there is little controlling the problem.