White chalky substance in exhaust

Hi,
I just noticed my truck is producing a significant amount of a white chalky substance in the exhaust. It is forming down the exhaust stack and even splattering back onto the front of my trailer. Any ideas what this could be from? The only new variable is that I recently used FPPF KillEm biocide additive to remove some algae build up in my fuel tanks, though I have not used it the last few fill ups and the substance is still producing.

Fluid levels, temps and pressures are all good
2020 Mack MP7 143k miles

Pictures
white exhaust substance


Thanks,
Pat

 

I thought about that but it’s much finer than dried def which is very crystal like and course. It is also an off white where def is a very pure and bright white

 

When you experienced it, it was a fine white, chalky powder? It was not course and crystallized like spilled def is when it dries?