I assume that the trunk is firm, not soft.

I assume that the white stuff is not mold.

It's probably just mineral residue from the water and/or soil. You can try filtering
your water - and try to avoid watering the trunk itself when you water. Also, use
rainwater as much as possible to rinse your plant.

My Cryptomeria has the same issue. I'm wondering if a mild mix of lemon and water might flush the calcium build-up.....or vinegar, perhaps? Don't try it unless you consult a professional.

Josh