How about soaking the plant in a bucket of water for 20 minutes and then using one of those hand pressure sprayers and set the nossle on stream and give the mud a good blast??

Are you saying that the trunk is simply discolored? I assume that you have been able to remove the actual mud. No? Probably just wait it out and in time and with exposure to the elements it will fade naturally. What's the hurry? There is also a product called Lime Sulfur that you might want to research but I am not familiar with it's use; it may not be suitable for all species.

Norm

Norm, it just has a line much like the "flood line" you see on trees along a creek bank. I've tried water and a soft brush. I know, good bonsa takes time. Zack